I think the project has a grander goal of being an open source base for new modes, weapons, and classes to be added to TF2, but for now is effectively quake-style deathmatch with a TF2 feel and weapons. Can be fun but very chaotic. Has autojumping so bunnyhopping is super easy to do. Is even crazier playing with all classes where you realize scout can bhop with the OF version of the minigun that doesn't slow you down when firing. I've seen at least one person criticize it for the lack of armor, but I haven't played enough quake multiplayer to know what difference that makes (Pre-Fortress 2 has armor, maybe that can be added here). Also playing this is how I found out the queen died when someone mentioned it in the chat.

Really high quality Source mod, it's arena shooter multiplayer but much more approachable and appealing. Good community servers and mappers means you'll always be seeing new stuff. Flamethrower feel out of place but they're working on a replacement as far as I know. Has a extensive options list for HUD and FOV which is helpful for an arena shooter, and cosmetics really let you personalize your merc. I'd say try it out since there aren't many other mods with as strong a presentation than Open Fortress.