for what it's worth:
DS and Diablo II are the only games I've played through twice. Partly because they're more action than plot, so a retread doesn't lose as much. I went back to DS later when some friends wanted to do MP, but I couldn't sustain interest past a week or so. DS is very much like a 3-D party-based Diablo II. If you're big on roleplay and storyline, go for the BG's and DD. Another thing I liked about DS was the ease of editing items and modding the game; I like being able to remake the game if I want (after going through the regular way).
I just started DD last week, and I love it. I had been slogging through the campaign in NWN, but got tired of it; NWN needs parties like BG had. DD has great depth and attention to detail; the only thing I miss from the 3-D DS and NWN graphics is the ability to zoom and rotate the camera. DD reminds me quite a bit of Diablo II, but with less repetition in graphics and a better interface (no melee clickfest) and much more involved quests.
Both DD and Morrowind are prime examples of how a solo RPG can be deep enough and have enough NPC interaction to make you not miss having a party.
