I have a one of those ergonomically designed brushes, but the bristles are getting pretty worn.
Here's two tips on getting carpets clean (both gleaned from information in this forum on this topic)
1) DRY -brush your carpets after the initial vacuuming to release any deeply imbedded dirt and vacuum again. This makes it much easier to
pick up DRY dirt with a vaccum, rather than WET dirt with your extractor of choice. (Shop Vac, Bissle Little Green Machine, or cotton towels if you don't have a commercial model, like a Mytee, Durmaid, or Sensei)
On floor mats, remove them from the car and use compressed air to blow them out and fluff up the fibers.
(Comon sense hint: do this AWAY from the car, so the debris don't get deposited on the car, both outside and/or inside)
2) If you have a Porter-Cable Dual-Action Polisher, buy a 4" round brush attachment designed for carpets and use that to brush your carpets.
Some detailers have the PC just for that reason.
I have found that a stiff-bristle toothbrush is great for corners, creavases, and edges where the interior plastic meets the carpet.