I started playing this a couple weeks ago, and I thought it was pretty cool. It adds new content and makes the game much more tactical. Most/all of the servers that run it enforce the mod's rules pretty strictly. The best part about it is that 95% or so of the people that play it will actually join a squad and actually work together. Gameplay is a lot slower, but I think it's more entertaining.
Any description I write can't really do it justice, so if you're interested:
http://www.realitymod.com/