As far as I can tell it's the same as the 05. I have been in either an Eldorado or Seville prior to my 05 STS since 1989 and all the back seats sucked. Shame for a car this size. If you want a decent back seat you have to go for the DTS, SRX or Escalade.
I owned a 98 STS and for a car that is 202 inches long with honda accord leg room is unacceptable. I sat in the back seat one time and let others drive and was not comfortable at all ! I have an Ls430 now and there is so much more legroom even with the seats all the way back! I don't get it
The dashboard is too (I can't think of a good word for it). I dont like it.
I havn't had a problem with space in the STS. I've had five people in mine before on long road trips without a problem (needless to say, we were all dying to get off, but thats standard with any road trip).
My dad and I were both in the front, and we are about 6' and 300lbs each. In the back we had people that were an average of about 5'8 and 200lbs. We were all fine.
Im usually the only person in the car though, so back seat space is of little concern to me.