Me and some mates were into this quite heavily, but go maybe once a month or so now. We did do the Mall once, it was pretty good fun, we managed to do one I no longer think operates, it was in an old victorian hospital and rather than semi auto only, full auto an no engagement rules , it was itense to say the least.
The modifying bug caught hold though, my M4 is running a shade below 350fps on .25 bb's but at 38 rps
My mates recently moved onto the air conversions which are awesome, very quiet and reliable and can be tuned to run whatever fps and rps they want within reason, not cheap though.
Its all about the location and the people that run it imho, we are local to groundzero outfit and whilst its a great site its a very commercial outfit, so loads of kids charging about not taking their hits and then the regular teams that always seem to have one ex military in who when you do shoot them get all stroppy because they have been shot and complain your gun is firing hot and get you pulled out the game to be chrono'd.