Although Dell lists the M115HD on its Web site as a home theater projector, it's actually an ultra-mobile designed primarily for business use. Like similar projectors built around an LED light source and a 1280x800 DLP chip, it's a prime candidate for giving presentations on the go. Not only does it weigh under two pounds and pump out over 400 lumens, it can read files from microSD cards, USB memory keys, or its 1GB internal memory, so you don't even have to carry a laptop with you. Even better, it's widely available for around $500.
The M115HD, like much of its direct competition, uses an unusual DLP chip designed to maximize light output from small projectors. Unfortunately, this chip also adds apparent scaling artifacts that produce unwanted extra patterns in large areas filled with repeating patterns of fine lines or dots. This shouldn't happen with an input signal that matches a projector's native resolution. But because they show up even when feeding a 1280x800 signal into M115HD, there's no way to avoid them.
You may or may not consider this a serious problem. For data images, you won't notice artifacts very often unless you use patterned fills rather than blocks of color in your graphics. However, the visual impact is to give the image a soft focus effect. For data images, this can affect fine detail in line drawings and make small text hard to read. For video or photos, the artifacts tend to decrease the actual resolution of the image, meaning the ability to resolve fine detail. Both of these issues show up with the M115HD, although not as badly as with some other projectors that use the same DLP chip.
Data image quality. The M115HD does a more than acceptable job on data image quality. Colors were generally eye catching in all predefined image modes. However, some colors changed significantly enough from one mode to the next that you might want to pick a mode based on what the colors look like.
Both blue and red, for example, were fully saturated, but a little dark in Bright mode. In other modes, blue shifted to sky blue or pastel colored, while red shifted to being brighter and distinctly orange-red. Color balance was good, with various shades of gray suitably neutral in all modes.
As expected, the apparent scaling artifacts were impossible to miss on any image with closely spaced patterned fills or fine lines. That makes the M115HD a poor choice for detailed line graphics, like an engineering drawing. The problem also shows in small text fonts, with both black text on white and white text on black readable at 9 points but hard to read at 8 points. This won't have any effect on the large fonts in a typical PowerPoint slide, but it limits how much of a spreadsheet you can show on screen at once and still read it.
Video quality. The M115HD's video is best described as watchable. After trying all the predefined image modes, I settled on Movie mode as offering the best color, despite some issues. Most notably, some colors in some clips were a little too intense, from a combination of being off hue and over saturated, even though the overall impression on most clips was that colors were a little washed out. On the plus side, the projector handled shadow detail well, and it did a good job avoiding posterization, even in scenes that most data projectors have trouble with. As expected, the soft focus effect from the chip architecture made the resolution look more like SD than HD.
One potentially annoying issue for video is that the projector uses a 16:10 aspect ratio for 16:9 input, which distorts the image. Typically, if a projector doesn't change its aspect ratio automatically, you can at least set it manually, but the only choices in the M115HD's menu are for Origin, 16:10, and 4:3.
Other issues. Rainbow artifacts, which are always a potential concern with DLP projectors, aren't much of an issue for the M115HD. I tend to see these red-green-blue flashes easily, but saw few enough in both data and video that its unlikely that anyone would consider them a problem.
Potentially more annoying is that the projector's focus changes as it warms up over about 10 or 15 minutes. This can be a particular problem with a portable projector, since you're likely to start your presentation immediately after turning it on. If so, you'll have to fiddle with the focus as you go. This could be a problem if you're not sitting right next to the projector.
Good Connectivity. The back panel of the M115HD offers only a few connectors, but they're well chosen.
1 24-pin port for a computer, paired with a 50" cable with a VGA connector on the other end.
1 mini-plug audio output
1 MicroSD card slot
A USB A (for a USB memory key, a $69.99 optional Wi-Fi dongle, or direct connection to a PC to upload files to the 1GB internal memory). The Wi-Fi dongle works with apps available for Windows 7, 8, and Vista, Mac OS X up to 10.8, and the iPhone, the iPad, and Android devices.
Setting up. One potential setup issue is that the M115HD doesn't come with a suitable cable for transferring files from a computer to its 1GB internal memory. If you want to take advantage of that feature, you need to get a USB A to USB A cable separately.
You may also want to get one or more of the projector's optional accessories, most notably the Wi-Fi dongle, the $24.99 remote control, or the $24.99 mini-tripod. Without the remote, you can use the buttons on top of the projector to give commands, but a remote can be more convenient.
The projector's vertical offset lines up the bottom of the image with the center of the lens. This is ideal for using a table-top screen on the same conference table as the projector, and will often leave you without needing to adjust the image position for a screen mounted directly in front of a table. However, if you need to adjust the image position and don't have the tripod, you can use the screw adjustment on the front foot to raise the image, and then square it off with the manual keystone control.
Setup is otherwise straightforward, with the focus tab offering just the right level of resistance for sure control. As is typical for this class of projector, there is no zoom. To adjust image size, you have to move the projector but that's not really an issue given the M115HD's small size. For an extended session in moderate ambient light, I settled on a 48" diagonal image, with the projector at a touch over 6' from the screen.
One minor issue for any small projector is that it is so light that it tends to pivot if there's any tension on the cables plugged into it. You may have to put something on the cable, like a paper weight, or tape the cable down to keep the projector pointed straight ahead.
Highly portable. The M115HD measures 1.4" x 4.1" x 4.1" (HWD) and weighs just 0.8 pounds by itself, or 1.8 pounds with the power block, power cord, and soft carrying case that it comes with. Even better, it can read photo, video, audio, and Office files directly from a USB memory key, microSD card, or its own internal memory, so you don't have to carry anything else with you. The HDMI port also lets it connect to any phone or tablet that supports MHL.
Low running costs. The LEDs in the M115HD are meant to last the life of the projector, so you don't have to buy replacements. Dell rates the light source at 30,000 hours.
Brightness. I measured the M115HD at 414 lumens in its brightest predefined image mode. The other modes, all of which offer somewhat better color quality, came in at 248 to 315 lumens. As already mentioned, I found the projector bright enough for a 48" diagonal image with moderate ambient light. However, you can easily get away with a larger image for short sessions or lower lighting conditions. I also measured a difference between white brightness and color brightness. The difference was most significant in Bright mode, which explains why red and blue look dark in that mode.
Good brightness uniformity. The projector did an excellent job maintaining uniform brightness across the screen, at 85% brightness uniformity. It's hard to see any brightness difference even if you make a point of looking for it.
Low audio volume. The 1-watt speaker in the M115HD is hard to hear from even a few feet away, making it useless if there's ambient noise or an audience of more than one or two people. If you need sound, plan on using an external audio system or headset.
No zoom. As already mentioned, the M115HD doesn't include a zoom control.
Like every other projector in its size and weight class, the Dell M115HD offers the advantage over pocket projectors of being a lot brighter and only a little less portable. It is smaller and lighter than many models with similar brightness, and brighter than many models with a similar size and weight. Beyond that, you can leave bulky image sources at home. The HDMI connector and optional Wi-Fi both let you use your cell phone or tablet as an image source, while the ability to read files from USB keys, microSD cards, and internal memory, can leave you with little or nothing else to carry. All this, plus good data image quality and usable video quality make the Dell M115HD a satisfying value for about $500 (see current street prices here).