I'm about ten hours in and so far I'm enjoying the game for what it is, but its got a few problems I didn't have with the first.
The enemy AI has certainly improved, as often times a bad guy will dive for cover when I start shooting at them, but nearly every time they've done that so far has resulted in them visually clipping through the barrier they are behind. It's really not very immersive combat when arms and legs are sticking out through concrete walls. I thought that technology was nailed down years ago (Gears of War?).
Melee combat is very clunky feeling. There is no visual interaction between my character and the enemy I'm attacking. There's just a generic punch animation, and if it connects, the damage number that follows it. Doom 2 comes to mind.
Framerate performance is... meh. For the hardware I'm running on (eVGA GTX 670FTW, Intel 2600k), I'd expect nothing less than a rock solid 60fps with the graphics on mid-to-high settings. Most of the time there isn't anything graphically intensive going on, but my framerate often dipped into the low 20s anyways.
Getting stuck has been a problem for me. I like to explore, but on foot, there are too many little crannies you can get to but cannot easily get out of, and the vehicle is even worse about that. With the wonky controls it'll easily wedge itself up onto walls and will have to be abandoned. It's bad enough that I just go on foot now instead. That was never an issue with the original Borderlands.
The UI is less intuitive than the original, and even that was just marginal - particularly the inventory screen. Picking a weapon from the list (that has to be scrolled through) usually takes at least two or three tries.
Overall, I'd say wait for this on a Steam special. It would've made a great DLC pack for the original, but I'm glad I didn't drop $60 on it. It simply lacks the polish I'd expect with that price tag.