Depending on what you like, anyway. The original BG was very true to that original tabletop RPG feel. It was more open than BGII, you were constantly trying to figure out what exactly was going on (in BGII/ToB, you are as well, but it's more OBVIOUS what was going on), and your characters were usually some what underpowered on gear. You had to succeed based on your MERITS, and you chose what you succeeded in. It was possible to play for days and barely advance the main story line, and it was all fun. There weren't just "side quests" there were "side stories". It was very true to the original concept of "RPG".

There was more STORY to BGII and ToB, I think, but please do NOT think that BG would not be as good just because it was a sequel. It has a very different feel. You're not a Child of Baal. You're an orphaned young adult trying to stay alive, make your way, make some gold, and maybe, just maybe, figure out wtf is going on. If you get around to it, anyway. Or you can make it a priority. It's REALLY up to you.

As far as mechanics, they aren't any worse. Seriously. They did not "improve" the mechanics for BGII. The graphics are a bit better (bit) aaaand that's about it. Some tweaks, but nothing drastic.

And for the love of GOD please don't compare BG to Diablo. The BG franchise is potentially the best RPG series in existence (and is definitely one of the best) Diablo is a repetitive dungeon hack.