It's ok. It's basically first person (or over the shoulder) "sci-fi" D2. It's just not nearly as polished, or balanced as D2. (even comparing release D2 to now Hellgate).
I played it for about 2 weeks, then went back to D2. Some reasons why:
There aren't that many unique area's or mobs, so you end up looking at the same layouts over and over, which does get pretty old. D2 probably had more variety here.
Like D2, it's real easy to hit a wall when going through a second time in the game (nightmare I think it's called). Depending on your play style, you're probably have to spend some serious hours grinding at the end of the normal mode before moving onto nightmare.
The single player version hasn't been updated in over 4 months. The MP is updated quite a bit (unless you live in Europe - if so, its been about 4 months as well).
The SP version is missing functionality and fixes that the MP has had for months. It also is buggy as hell. Expect crashes and other wierdness in SP.
Both versions have a memory leak that'll do weird things after awhile.
The MP population, which was never very large has dropped a bunch since release.
There are limitations on the number of MP characters you can have, and the number is pretty low if you aren't a paying subscriber.