we used to race Supras in the old Speedvision Cup in both turbo and NA form, and pretty much they are the same as the IS300 under the skin. I also used to know someone that raced an IS300 in a professional series as well, but it was not exactly a stock class. Anyway, point is, they are unbelieveably tough cars. They simply don't have the same wear issues the BMW, Mazda, GM, or Ford's used to have that we raced against.
We basically did nothing other than maintenance, nothing. A whole season on a single clutch, no hub issues, easy on brakes, no transmission issues..etc. These are all items that used to be consumed on a regular basis by the other guys. I'd say the IS300 is one of the last of the bulletproof Toyotas.
That said, i own a G35 and love it. I get about 22mpg everyday in town, is fairly fun to drive, and feels kind of like a hot rod really. It's also very comfortable and nice, and things just work. It is very, very different than my previous BMW experience in that regard. Although I still love them too. It is an automatic though, althought it seems to suit the car well. Not sure I'd want a stick in it after living with mine for a year now. I have other cars for that anyway.
You really need to go drive a few cars though and see which ones you like best. Like you, I have a bad back, and not all seats work for me. I've had some people tell me that car "X" has the best seats ever, only to leave me in pain after a few miles. It all depends on the person I think.