When I carried a Glock 22 daily, I used a KSC G17 Gas Blow-Back (Link Removed) for training. It was great because we set up targets in the office to train tactics, such as entry\team entry, hostage situations, multiple targets, force on force (inside) etc.. things you can't do with live ammo or simunitions easily. It fit my Safariland 6360 just as well as the real thing did, has similar weight and handling, and when loaded to 15 rounds provided realistic capabilities. Now that I'm a "civilian" and my EDC is a Springfield XDM, I use a Tokyo Marui XDM (Link Removed) for indoor training.. and honestly if I put the two together I can't tell them apart. There are also cheaper models made by WE (kind of like Nissan is to Infiniti I guess: same company), but if you plan on these seeing heavy-duty use I wouldn't spring for the cheap stuff.
No, I'm not one of those "super-tacticool" Airsoft players (wish I could find some people to play with though, I miss force-on-force training), but I do feel a good quality one can provide some great training and allow you to do things you can't do with live ammo or even simunitions - I also realize there's absolutely no substitute for true trigger time. And hey, on a side note, the Army uses Airsoft for training as well.