Mine get pretty toasty on high, I can definately feel them through my jacket.
Funny thing is I bought the car without knowing it had heated seats. I show up to pick it up at Hartford CT airport and it's about 0*F, I sit down in the car and "WHOA what's that." He says "oh did I forget to tell you? It has heated seats."
Before I bought my Seville, I used to mock cars with heated seats. I always felt it as an unnecessary option. After having them this past winter, I do not know how I ever drove without them. Definitley one of the nicest things I have ever had in a vehicle. I love my heated seats.
If you hear a clicking sound after activating your seat, and the seat doesn't warm up, you probably have a break in the seat element. Do a search and you will find the correct part number is #25755077. I have purchased the part but have not yet installed it, so I can't give you an opinion of how hard the job is.
I have a 94 Eldorado and the seats get really warm (hot) on high. My wifes Volvo doesn't really get warm enough to do any good. I used to laugh at the idea of heated seats but now in mid winter New York State I really appreciate them.