Sound quality is only gonna be as good as the worst component in the system.
So if you're running cheap speakers, it's not gonna make a lot of difference anyway.
However, if you go to some decent speakers (£100+ a set) then you really start to notice the crispness of the better HUs.
Sony, unfortunately, seem to pile all of their development into making the displays do all sorts of fancy crap that realistically isn't very useful anyway, as you should really be having your eyes on the road!
My mum has a £50 shark mp3 cd player off ebay, and it sounds just as my £190 alpine does in her car - but she has standard speakers, so not much ground for comparison.
When I asked my friends why they bought sony headunits, the standard reply i got was "cos they look cool".
Me and my car audio homies all have alpine, and with good reason. Sound quality where it matters!
I should be getting my new 2005 model alpine 9853R sometime on Tuesday - can't wait!