I used one for about a year and half. To echo Jade's comments, it takes a fair amount of tinkering to get it as smooth as a CCM, but is definitely functional.
I had to swap pump kits twice (WGP -> WWA -> CCM). And you have to be careful which frame (plate) combo you will be using. I tried a DYE frame, which was too narrow for the supplied post-98 trigger plate and wound up using a WGP composite. Setting the lug release for A/T was fairly simple on mine, but the pull was not as smooth as a CCM with a noticeable "bump" as the sear released. FWIW, a friend of mine had a real hard time timing the release on his own marker with GAT kit to reliably autotrigger.
I've used CCM kits both before and after the GAT kit and prefer them for both ease of installation (true drop-in) and smoothness.
I have several Sniper 2's with slider frames, but wound up selling my GAT equipped one in favor of the non-A/T slide markers. If I really feel the need for A/T (rarely), I'll break out the CCMs.