This is just my opinion, but things do get better with price. Of course there are expensive phones that suck (Ie the upper sony line, excluding the 3k, r-10, etc)
You got to remember that phones heavily depend on a good source and amp (and recording). Many people buy HD650s and RS1 for instance, and use them with either bad or low-end equipment (ie mediocre portable source). This is just a waste, trust me, I've done the same thing. ($$ phone, cheap source/amp -> acceptable sound) You'll never know what they can really do.
I really don't think a low-end phone can beat a respectable "giant", when it all the components are good. Low-end phones are of course more forgiving, but that doesn't necessarily make them better over all.
But if you're happy with the basics, go with it. There is no reason to spend more if you're happy. Some people either hear small differences with better stuff, some hear big ones. Many people downgrade from the good stuff, because they want to. If you can't appreciate the benefits of a better rig, revert. I myself am f**kin' crazy (seriously), so I like listening to better and better phones. I have an entry-level setup and enjoy it greatly, but I will always desire for more. For a crazy headphone audiophile, only the best (we can afford) will do. But I doubt giant killers will become the staple of head-fi.
Besides, if an (for example) ipod, supermini, and low-end phones were truly the end-all of hi-fi, why do people have expensive rigs? Because we care about great sound (as opposed to acceptable). Not to mention most of us are probably nuts.
(please don't flame me for this)