Baldur's Gate is a blast to play and has some classes and abilities that I haven't seen in any other game. The only big problems I have with its gameplay come from using 2nd Edition D&D rules. Magic seems to dominate the game to such an extent that you can literally magic missile dragons to death without much effort, IIRC, but it's still fun even if you're not using a lot of arcane spellcasters.
However, I don't like the way BG II forces you along certain paths and (spoiler)
forces good and neutral characters into making a certain set of decisions in the Trials before the fight with Irenicus. If you don't make these exact decisions, you turn neutral evil, even if you've done nothing but save innocent people and fight the forces of evil for the entire game.
(end of spoiler) You can make some decisions, like which group to join at the start, whether the protagonist's personality is basically good or evil, and in some cases how to finish a quest, but the plot isn't nearly as sensitive to your decisions as Torment's is.
Even with all the complaints, I think it's a good story. It might not be on par with Beowulf, but it's fun and interesting enough for me to want to see the end of the game.
The only other thing that really bugs me is the dullness of some of the NPCs. I really like playing with some of them. Minsc, Jan, and Lilarcor in particular have some hilarious dialogue when placed together. Aerie has some funny interactions with other characters if you can put up with her. But there are too many characters that don't seem to have any development or personality to them. Some of them are required for a couple of the game's more interesting side quests, which makes their dullness even more annoying.
Planescape is very different from Baldur's Gate. It has one of the best storylines I've ever seen in a video game and an equally good cast of characters. It's also very low on combat, at least in my experience playing a mage with nearly-maxed dialogue-related stats. I've been told that it's more action packed with a fighter, but I doubt it's ever as combat oriented as Baldur's Gate 2 or Icewind Dale.
I love the weird areas and enemies the game makes you fight. If the combat was as fun as it is in Baldur's Gate, I'd probably replay it more often than any of my other RPGs. The variety of armor and weapons is tiny and the characters don't reach very high levels, making combat a little unexciting for me.
The strong points of both games are more than enough to make up for their faults.