I have used GSW for doing greenscreen stuff at mess functions, and as a back up for my primary event software that has a GS function.
GSW is very powerful software, it does require a half decent PC though.
The suppliers are pretty limited, it can be bought direct from the company though, or via 'photomart'.
Tips for use:
Use a decent green or blue backdrop, the above company do a huge 3mx6m green for about £50.
Lighting- this is key, shadows etc can be dealt with by the software, but the less shadows the better the result.
If using for full length, the floor will always be an issue, it will always look iffy.
On the whole, good software, a dedicated solution that is a bit clunky for fast paced events- but worth it for when i have someone wearing something close to chromakey colours (such as a borat mankini!)
Busy trying to sort out the shite caused by some real slime on the main site - now working more on things that will benefit me, plans that I meant to put in place for some time - hence trying to pull together the mess photographers network.