Things I liked: Fits the Umarex's Walther PPQ CO2 pistol like a glove. Very good & strong fit and does not wobble, yet easy to attach/remove. Easy to calibrate my Walther pistol scope. Feels like a natural extension (which cannot be said of any other acc rail I've tried in any as/bb pistol so far). Bottomline: The gun is wearing this rail, it's not hidden inside it.
Things I would have changed: Nothing, really. A bottom rail would make this thing too big for my taste.
What others should know: Only fits on a PPQ-shaped design; does not fit any other P99 models (Umarex/Walther/Maruzen), they don't have picatinny rail 'teeth' and the overall shape is a little different (about 1mm too wide below the slide). If you want to fit this on a newer P99 model, you need to carve off lots of stuff from the pistol, since this acc rail is metal.