I've been more than satisfied with the stock system in the IS. Other than the 6 disk jamming up and breaking.I was pleasantly surprised with the factory pioneer in my scion. I've never been a fan of Pioneer or factory radios in general. That's the first time I've left one in place.
Also, I very much like not having to worry about getting my damn stereo stuff stolen (although some fuckers STILL stole my damn wheels off the thing).