For a long time, I was planning on Thief primary. I'll do my usual altaholic thing and make all the classes, but Thief was the one I was looking forward to most.
Then they dropped Energy, but left Initiative. While the version with separate ini bars per weapon was interesting, that got reverted. I'm just not interested in Thief as much anymore.
What I do like, to my own great surprise, is Engineer. While you do need to use Utility spots to give yourself weapon swaps, the combination of decent skills, turrets, and the backpacks, PLUS all the toolbar awesomeness makes this class seem wonderfully versatile. I also love the idea of two guns dealing out a ton conditions then Rocket Kicking anyone who gets close.