I'm sure you would have rather heard from a non-modder, BUT:
If you poke around, there are actually people discussing a number of the mods. For instance, in the 8 months or so since its release, The Darkest Day's merits and demerits have been beaten to death (some people like the new areas, new quests, other people don't like its incompatibility issues, its relative lack of attention to detail like fleshed-out NPCs and writing, its power level, etc.) It remains officially compatible with SOA-only although a TOB-compatible version is in the works.
The mods that I work on and tend to be the most interested in are NPC add-ons, because I happen to feel that above and beyond the core storyline or the side-quest, "friends, companions, and lovers" are what made BG2 a great game. Having written Kelsey, I will shamelessly but honestly say that he works with pretty much anything: plain SOA, TOB, TDD, and all the other NPC mods, as well. Using Wes Weimer's WeiDU utility (which rules more than Tiax), it's a very do-able project to drop in any number of new NPCs, quests, etc. without conflicting with other projects.
So Kelsey, Tashia, Solaufein, and Valen, which all use the WeiDU system, basically all inter-operate and play nice with each other and with the rest of the game. Arnel's Nalia is a bit of a different story, while it works with some other mods it is trickier to install.
The other "big-name" mod would be Ascension, which extends and enhances the TOB end-game. It was lead-developed by David Gaider, co-author of TOB, and implements things that were cut or under-explored or just "how he thought it should have worked."
Kelsey lives at
http://www.forgottenwars.net/kelsey
You can grab WeiDU, a new WeiDU version of Ascension, and Solaufein from
http://www.cs.berkeley.edu/~weimer/bgate/
[ 06-02-2002, 10:26 AM: Message edited by: jcompton ]