If you want to go somewhere to mess around and not take it too seriously, then it's okay. There's lots of running/walking about since it's an open space. There are lots of problems with the site and it's history. They don't chrono any guns, they say they do random chrono checks, but I've never seen them do any. A kid also lost his eye there a few years ago because of their non-existent safety rules at the time, and their rules aren't much better now.
Here's a news article:
https://www.sloughobserver.co.uk/news/17601363.iver-teenager-blinded-bb-gun-airsoft-game-marlow-launches-200k-compensation-claim/