You really can't beat Reaper for most character miniatures or specialty monsters. They have familiar packs too, and other small things like that. I was going to buy one the other week.
The D&D Mini's are good for those D&D specific creatures, though Reaper has a lot of them as well. Check eBay and you can find lots of D&D monster though, I bought 20 Kobald warriors, 2 large Air Elementals, & 6 Lizard men for about 12 bucks plus shipping in total. The only thing I don't like about the D&D mini's is the sometimes inconsistant scale across sets, and the paint jobs which are adaquate, but really look poor compared to a moderately well painted figure. I enjoy painting figs but time for the next few months is tight so I can't paint as much as I want to. However there is nothing better than seeing the players jaws drop when I whip out a new monster that I had painted and put it on the mat for battle.
So my view is paint the important stuff and buy the small stuff from the D&D sets.