Low gain. In ambient light, image brightness is always a concern, especially when you want to use your projector as a TV replacement. As such, the XS850E's low 0.85 peak gain makes it more difficult to obtain a sufficiently bright image. If your projector has a brighter image mode (often called Standard, Living Room, or Normal), you might consider using it to boost light output and image brightness. On projectors without this option, the screen's low gain can limit screen size.
Then again, having a large, bright picture on the wall in ambient light while maintaining contrast and black level is impossible without the use of a screen like the Draper XS850E. There are some other ambient light screens that do have higher gain ratings, so those might be an option if your projector is not capable of producing enough light.
Price. At large screen sizes of 80" diagonal and above, the Draper Onyx with XS850E makes good financial sense. It provides a TV-like experience at an image size where equivalent televisions typically cost more than the projector/screen combination. It also allows you to upgrade the "guts" of your display periodically by replacing the projector rather than the whole system. With a 100" diagonal screen priced at $2,717, the total cost including projector can be less than $6,000. Draper can manufacture seamless XS850E fabric to a maximum vertical size of 62.5", allowing for a 16:9 image of about 125" diagonal.
On the other hand, a 62" version of the Onyx with XS850E still costs $2,125. At this size, the screen is already at a price disadvantage to similarly sized televisions, and that is without including the cost of the projector. At these sizes, the price advantage of the Onyx/XS850E disappears. In this case, bigger is better.
Projector placement. The XS850E fabric has a wide half-gain angle in the horizontal axis, but a narrower one in the vertical. This is by design, as it vastly reduces the amount of ambient light reflected from overhead sources. But if you have your projector in a ceiling mount at an extreme angle, such as from a super short throw projector, the ideal angle of reflection from the screen -- the "sweet spot" -- will not be straight back towards you. That can make it difficult to get the brightest possible picture out of the XS850E. The ideal projector mount for the XS850E is either a ceiling mount towards the rear of the room or a rear shelf mount.
Rear ambient light. The XS850E is normally very effective at rejecting ambient light, especially from overhead or side-lit sources. But there is one circumstance when the XS850E (and any ambient light rejection screen) offers no real benefit, and that is when the ambient light is coming from the same direction as the projector. In other words, if you have a rear shelf mounted projector with a window directly behind it, the screen has no way of differentiating between wanted and unwanted light. This is not a flaw with the XS850E, but a limitation of all light rejecting screens which there is no easy way of correcting.
Draper's XS850E fabric is a high-performance ambient light rejecting screen material. It does not fall prey to some common problems of ambient light rejecting screens such as narrow viewing angle and difficult assembly. It does have a relatively low 0.85 peak gain, so a bright projector is needed to realize the screen's full potential.
The Onyx with XS850E still carries a price premium over many conventional projector screens, but it does things that those screens cannot do. It represents a good value at larger sizes, especially 80" diagonal and above. Smaller screen sizes are less appealing due to the increasing affordability of large-screen televisions. The screen succeeds as a solution to daylight projection in rooms where ambient light cannot be fully controlled, and the experience itself is compelling. Paired with a bright projector, the Onyx with XS850E is a great way to bring the 100" TV experience to your living room without breaking the bank.