Zombie Panic Source is a very fun game. It pits zombies and survivors in a PvP style set up. The zombies themselves are very powerful- they’re quick, can jump, climb ladders, and they do a lot of damage in melee range. The survivors have an advantage in that they can use ranged weapons, but that’s about it. Although different servers provide different things, the common underlying gameplay, in terms of objectives, is that the survivors will be A to B goals to complete (objective maps), or will be put in some kind of arena where they have to kill so many zombies to win (survival maps). Some maps, also called survival maps, will put survivors in a defensive position where they have to hold out against a zombie horde with unlimited lives.
According to Steam, I’ve sunken about 600 hours into ZPS- and for good reason! It’s a very fun game in which each match is a different experience. However, it is very difficult. As I said, the zombies are very fast and do a lot of damage quickly in melee range. Although you have a lot of different firearms at your disposal, each zombie will take about a full magazine to kill with body shots (although skilled headshots will allow you to down 2-3 zombies with a single magazine). This is further complicated with ammo usually being hard to find. Although melee weapons are available (and a lot of them), melee is hard to get skilled at. The majority of rounds I played usually end with a zombie victory.
Would I recommend ZPS? Yeah, I would. It’s a free game with a lot of community content. I’ve made some good friends playing it, and it’s just a good time all around.