I'm playing Borderlands at the moment with a friend and am loving it!

But I need another game to play single player so what would people suggest in the Action RPG niche?

Gears of war is another one of my favorites because of the game-play but doesn't have the RPG aspect... Also just finished splinter cell and am looking forward to the next due to the strategy involved... can be frustrating sometimes though!

I used to be into fantasy type games like Diablo but haven't found any to be all that great on 360... yet...

Deus Ex(the first one, not the second one) is one of the most highly acclaimed Action-RPGs there is. It is rather slow paced, but has a branching story line with lots of choice & consequence, and is deeply philosophical.

Vampire: Bloodlines - the Masquerade is probably one of the best games I have ever played, but make sure to install the unofficial patches since it's buggy otherwise

Outcast, while it didn't have level ups and such, had an open world with lots of sidequests, but it's hard to find nowadays, and hard to get working on modern computers

The Witcher. One of the best action RPG games ever in my opinion.

Mass Effect 2.

Also check out Star Wars : Knights of the Old Republic and its sequel. Its more of a RPG than an Action RPG, but it's a very good game nevertheless.

I'm going to try mass effect 2 as I'd never heard of it. Thanks for the heads up.

Mass Effect 2 is an excellent game, with good action and story.

Mass effect 2 is way, way ahead of anything else on the market. Get that asap!

I got it in the middle of playing Borderlands, and I never touched or finished Borderlands again after that. It is just that good.

Dark Messiah of Might and Magic. On the PC.
Trust me, if you give it a decent chance, you'll go home pleased, at the least.

If you never did any classics, Diablo 2 is also a game I might reccomend.

Also, I'm not sure what to categorize it with, but I'll post it, just incase you're interested:
Overlord 1 and 2. They're more of a hybrid of RPG, Action, Strategy and simple puzzle games, but they also have potential!

I'd suggest Morrowind. It shares a cool kind of weirdness with Zeno Clash. It has a sandbox world which is huuuuge. With many factions hundreds of quests and a huge detailed amount of lore.