My stock (carpeted) mats got dirty faster than my car.
I bought the rubber Subaru mats for the WRX.
They work great, but are way too small and scratch up in a couple of months.
Now I just bought the R Racing Stage II floor mats. They look great (in gray), stay in place and cover so much more of the floor than any stock mat. At $24 at Schucks, it seemed like a pretty good deal.
I'd be curious to see what everyone else uses. This might help a newbie get the best mats the first time.
They also come in blue!