"so the obvious question is...is the game worth getting if i already have diablo?"

Yes. Diablo is focused on getting better equipment so you can kill more monsters so you can get better equipment. I found it fun for awhile, mostly due to the multi-player. Divinity has greater variety of quests, humour, story....

If you get the game, you can extract the music files to listen to whenever you want;
Divine Divinity Music Extractor by BrainBox.
also;
Divine Divinity music
Merry Christmas and Happy New Year from Larian Studios!

The latest patch fixes most of the bugs and makes it much more stable, though without knowing exactly why the demo was crashing (which was based on old code from before the game's release) there is no guarantee that the game will perform flawlessly. You probably will not have a problem, though.

There are only the three skill sets, for a total of 96 skills. There are differences in how secondary stats are calculated depending on class;
differences between classes (spoilers)
or
Early Starter Tips (NON-Spoiler Version) - contains much of the info in the above topic