If you don't need the Zoom for space-saving purposes, don't bother getting it. It won't help if you've already got ample area. It basically works by squishing and widening the playing field so, where a single-player game used to work optimally at about seven feet (nine feet for two players), it now functions well at four feet (six feet for two players). The playing field now ends at around eight feet with the Zoom, instead of ten feet without.
Games we tested (Dance Central, Your Shape and Kinect Adventures) worked as intended with the lens on. An awkward side-effect of the lens is that it does create what appears like dead space around the perimeter of your feedback screen (viewable on the dashboard and in some games), but if your body moves into those spaces it will still be recognized. The Zoom also creates slight distortions in the player's silhouette; again, the system still recognizes bodies and creates the correct vector skeleton of the player, it's more an issue of just noticing that it "doesn't look right," instead of it actually affecting games.
If you live in cramped quarters and the Nyko Zoom will let you properly play Kinect (or just avoid moving the furniture around to do so), the attachment is worth the investment.
Update: Although the lens did work for us with the games we tested, we are receiving and reading feedback that is making us uncomfortable about recommending this product. At this time, we will simply say that the Nyko Zoom is available and results vary.