I've own the W950i since last November, wow nearly half a year already!
From the stock earbuds, they are bass heavy but not much else.
Then I got a pair of v moda vibe, was travel around Europe for a month using them. It really made a world of difference, I had the eq set to dance and I really liked it. Mega Bass is crap, the mobile can't really handle it, and distorted the sound.
However, the vibe was probably too sensitive and was picking up some background sound (another weakness of the w950i). So I give the vibe to my dad and got a pair of sony ex91sl.
The ex91 solved the background problem, but again a little bass heavy in traditional sony fashion, but at least the now mids are alot better now.
But back to your original question, the sound quality is probably as good as a 3G ipod or the 1G nano with stock buds, but as soon as you move up the price point (headphone) you will find the w950 is let down by not having a dedicated sound processor (it runs a ARM9-based Philips Nexperia PNX4008)
As a mobile+DAP there is no better at the moment till the iphone comes around.