I have only played one complete SP game, which I did a year ago. I made a beeline through the story so I had left nearly all the sidequesting for future games. I then played all the Durlag's quests in SP. Shortly after that I got into regular online MP games with other players. That was the only way I played BG for the better part of last year, although in none of the campaigns I was in did we ever get around to finishing a game! Then this fall I decided to try playing a number of SP games with Dark Side installed using parties I had always wanted to try. Now I'm playing in an MP game again with one other player that focuses more on the BG city sidequests, most of which I have still never done.
I'm really amazed at the replayability of BG1/Tales. The most recent things I have been doing that have refreshed the experience of the game are to play in a speed game competition (using a no reload rule--very difficult but fun!) and roleplaying characters in online MP play with another player. The address to take a look at the speed game competition:
http://pub20.ezboard.com/fwanderonswarrensfrm12. (It's a really fun challenge in it's own right; I'd recommend just trying it even if you don't 'enter' the competition--but, hey, if you do succeed then then what the heck, why not submit your effort?)
I have gotten well into chapter 2 of BG2, both in SP and in online MP with other players, but for a number of reasons I just have not been able to get into it to nearly the same degree as BG1/Tales. In a month or two I plan to give SoA/ToB another go in MP where we will continue roleplay our characters (although perhaps we'll make new characters).