I can't believe you've never played this, Sir G.
*scratches head and thinks*
That's right you were shackled to that Mac for a while weren't you!
Everyone's pretty much covered it. I found BGII better from a power gamer standpoint, it's more of a hack and slash as compared to BGI, but from a pure roleplaying (for it's time) BGI is a far superior game than BGII.
Yep, the graphics are outdated and can only be run at 640 by 480 (which really blows on a 22" monitor) but the story is superb, and very non linear (compared to BGII)
It's a real pain to have to reload a few times to get past half a dozen kobolds, but it forces you to think more about tactics and using your NPC's in battle effectively.
On a scale of 1 to 10, I'd give BGII an 8, whereas BGI scores a 10!