I've also got the 15-85 and love it, I just don't use it as much as I should these days, because I'm not really travelling.
In short, the 15-85 is a 'travel' and 'outdoors' lens, the 17-55 f/2.8 (or equivalent) is better for indoors and at night, the IQ on both of them is past the point of "good enough" for most cases.

I've ended up with primes that overlap it in that range, the Samyang 35/1.4, Asahi Takumar 50/1.4, EF 50/1.8 II, EF 85/1.8, and if you've got the cash I'd definitely recommend the route of the 'travel' 15-85 and 'low-light' fast primes.
If I'd gone with the 17-55 f/2.8, that's great for low-light, and it may have been the only lens i'd gotten and not bothered with the f/1.8 primes, you only gain a stop and a bit from f/2.8 to f/1.8. But going from the EFs 15-85, f/5.6 to f/1.8 at 85mm is a ridiculous improvement. Rarely do I want anything faster at 85mm if i'm out travelling or hiking (where I'd only bring anything wider than the 70-300 for landscapes anyway, when I'd stop down to f/8 anyway).

So the 15-85 may be a 'do-it-all' lens in one sense, because of the focal-length-range, but in another way the 17-55/2.8 is more 'do-it-all', because you can take portraits as well as landscapes with it, and you don't "need" primes as badly as you do if you go the 15-85 and want to take portraits too (and can't justify extra cash for the amount of times you take them).

And to agree with kayaker, a flash is very useful no matter how fast the lens. I spent most of xmas day with my 7D with 430EX mounted on the top with cheap $5 chinese ebay softbox, switching between the samyang 35/1.4, takumar 50/1.4, ef 85/1.8 taking shots of kids running around unwrapping presents, using the flash I kept it on M at iso100, 1/200s, f/2-2.8 or so for extra DOF, and let the flassh fill in any extra holes where it was too dark.

So definitely, think about your use-scenarios, if you see a bit of indoor use with kids running around, and only have limited money ("this will be my only lens" scenario), the 17-55 f/2.8 may be the way to go. If you're going to travel and/or want a lot better quality in low-light, go the 15-85 and get primes later. Or go the 17-55/2.8 and get primes too, but you're missing out on the widest end and may need an 8-16/11-16 or so further down the track.