Possibly the best computer rpg I've ever played, and that's going against some very heavy competition. I'm not sure how something as amazing and weird like this was ever greenlighted let alone made, but I'm glad it was. If you loved Planescape: Torment, chances are you'll love this. Maybe even more.
So far, still in first half of game, this expansion rocks. Easily better than Baldur's Gate (1), and at times reaches BG2 levels. Great plot, good writing overall (some great bits, some slightly awkward ones). Some clever puzzles and so far one really intense sequence which involved both combat and puzzle-solving, on a (loose) timer. Voice acting on par with BG1 (I just replayed BG:EE in its entirety, so can compare). The new companions are fun; I expected to go with the old standards but am actually using new ones quite a bit here. The UI tweaks are improvements (though the UI and engine are showing their age, of course). So far (on OS X) zero bugs, though I do expect to run into a few at some point -- this game only released a few days ago. I understand there's a patch incoming pretty soon. Overall, if you're an old BG fan, you'll love this. This is much better than the first DLC (Tales of the Sword Coast). People who haven't played the original game will probably want to give this a skip -- much of what's great about this (and BG) is tinged with nostalgia, and BG2 is still the (much) better game.