I came into the headset market certain that I wasn't going to spend more than $150. However, I too was expecting a high quality and flexible bit of kit that I could use. What I found, however, was that this is a somewhat unrealistic ambition, due to the fact that I was probably only going to buy one headset, so I wanted to get it right first time. There are a few decent headsets for that price (with the Tritton AX720 - I believe being among the best). One concern I ran into that you may also touch on is the stereo vs 5.1 headset issue. After a LOT of research, it became clear to me that stereo was the way to go for both all-round and gaming performance (when combined with the right technology). I haven't tried any 5.1 headsets, so I can't personally contrast performance; but again, this is what was evident to me after a lot of lost hours on the web.
So, with quality, flexibility (of practical use, not design) and price in mind, I ended up with an Astro MixAmp. I also got a pair of A40s (which I use often for, among other things, LAN gaming), although I consider them to be rather overpriced. I've heard that a 5-series pair of Sennheisers will do a better job, for less money (but without the microphone attachment - a problem easily solved on the PS3, mind).
The quality jump isn't noticed too much in certain areas (say PES2010), but there are times when the quality really shows (such as online FPS gaming - I can really pick out enemy footsteps, watching movies and certain types of games such as MGS4 and Dead Space). Combine the quality with the diversity, practicality and price of my 'Astros' (especially the MixAmp), and I'm a pretty happy gamer. I love the fact that I also have an excellent pair of headphones for listening to music with. I just switch off the Dobly Headphone feature, and relax to a bit of Radiohead, or Samuel Barber... Like I am right now!
But as I said, opinions seem to be swayed toward the MixAmp, with a pair of Sennheisers for the best value on PS3 (though even they - plus a MixAmp - will still set you back roughly $250).
If, however, that's out of the question, then Trittons (PROs and AX720s) and Turtle Beach X41s seem to be a good compromise between quality and price; but are really only for gaming and movies. I hope that helps!