I've never been able to convince myself that the Stella McCartney bags are worth the price. For those prices, I want leather. That's just me, though.
As for Steve Madden quality, I've only ever owned one of his bags -- purchased on a trip after my bag was destroyed (well, ruined for me anyway). It was a big, slouchy hobo of made of a dark teal "snakeskin" PVC. I bought it because it was inexpensive (maybe $60), the right size to hold my stuff, and went well with the clothes I had with me. I ended up using it quite a bit the first year that I had it, and less frequently later. I finally sent it to Goodwill in a big closet purge about five years after I purchased it. It was still in good shape, and looked barely used except for some light pen marks inside of an interior pocket. I definitely got my $60 of value out of it...and then some.