I haven't gone too far with thief, so I couldn't tell you how I completely feel about the class until I get more skills unlocked.
Engineer is fun as hell, the class offers extremely versatile play styles.
I took my PvP build and implemented it to PvE and it's been very effective in dungeon runs.
Basically a turret build with equal points distributed to firearms and explosives.
If you ever go Engineer, let me know and I'll share the build.