I found that Gap Maternity was pretty dead-on for sizing (reg vs maternity). Some things did run bigger, but toward the end of pregnancy - when everything is bigger, not just belly, that extra room is nice ;-) They've usually got great clearance items, also.
Target (I think it was Liz Lang and another line) seemed to be HUGE. Tops were a little more like regular sizing, but bottoms were difficult. Again, in my ninth month, I finally needed that extra room, but from about 6 month until then, they just looked like giant pants/shorts on me.
Old Navy was ridiculous. I actually just bought my regular size in their regular line for a lot of things.