I'm having fun with it. Got to level 15 today, just playing by myself. I wasn't sure if this would be a game that's enjoyable alone, or if you need people, but I was satisfied being by myself. I did join one game online just to check it out, and it had some pretty bad lag. Hopefully it was just that game though, and it's not always like that.
I'm more interested in getting it for the PC however, for the controls, better graphics and smoother frame rate. As well as a modding community, because it seems like a game that will thrive with some great user made content.
And it feels very well done for an FPS game. Some times games like this focus on so many other elements, the FPS aspect isn't as polished as it could be. I love Fallout 3, but if you aren't fighting in VATs, the FPS gun play is a bit, unrefined. It just doesn't feel right trying to run and gun, but I guess that's not really how you're supposed to play. This game definitely feels good control wise.