I just acquired the 55-250 is yesterday and I am BLOWN AWAY by this lens. I have a 50mm 1.8, 28-105 USM and an old (pre-USM) 70-200 from my 35mm days, and the new lens puts them all to shame for IQ clarity and sharpness. Yes, faster would be even better (although I'm sure it would up the price), yes the AF may hunt a little sometimes and it's noisier than USM and yes, it's plastic (which I consider a bonus rather than a drawback for the more casual user, since this thing is beautifully light and well-balanced on my Rebel XT) but for the price? It's a winner - the IQ on it is stunning. The IS seems to work fine - I've experimented a little with it, and so far, so good as far as I can tell; even without it on, the pictures are stunningly clear.
I wish this lens had been around last year when I first got my Rebel XT - I'd have skipped the 28-105 completely and gone with this and a "thrifty fifty" instead. As it is, if the 18-55 is anything like its longer-reach sibling I will definitely consider trading in the 28-105 for one.
At last, the kind of IQ I've been looking for in my price range!