I have the 15-85mm lens, and it is a great lens. I rated it excellent- because for what it is (a variable aperture, nonL, EF-S lens) - I believe it is closer to 'excellent' than just 'good'.
The 15-85mm lens is my main lens on my Canon 7D, but I've also used it on my first DSLR (Canon 350D) - and it performs very well on both. I have no doubt it will do well on your Canon 550D.
Initally when I bought my Canon 350D, I had the kit lens (non IS 18-55mm) - which can be quite decent stopped down. But the 15-85 is better (sharper, more vivid colours, superior micro contrast).
USM focussing is the way to go. Much faster, but also more accurate (particularly in low light situations). The 15-85mm IS is great, I consistently get 4 stops hand-holding advantage (I can't compare it to the 18-55mm IS as I have the older /non-IS version).
The build quality of the 15-85mm is great, 2 decent steps up from the 18-55mm. I have a good copy of the Canon 28-135mm. But the 15-85mm is one step up again from the 28-135mm both in terms of image quality and build quality.
The 15-85mm is not quite as good as my Canon 70-300mm L nor the Canon 100mm f2.8 macro in terms of image quality, and certainly the L has again superior build quality.
However the 15-85mm is my 'go to lens' for general photography. It has great range (I found the 18-55mm range too limiting - particularly on the tele range). The 15-85mm is generally very sharp corner to corner at nearly all focal lengths and where it is not 'very sharp' it's 'sharp enough'. Handles flare well.
Hope you'll do well with your decision. Let us know - and share some photos!