Why not you also consider EF 70-300mm F4-5.6 IS USM or EF 75-300 F4-5.6 III USM?
Because I think these 2 lenses will better complement your current 17-85. The focal lengths will overlap a little, but not as much as 55-250 to amount to a waste. Plus with USM, like your 17-85, you'll get fast and quiet focusing.
I've tried the 55-250, it is a good and decent lens, but sharpness is poorer than 17-85. And like others have said, it will not work on full frame bodies.
I've not used the 90-300mm before, so I can't comment, but if you buy that lens, you will be losing 5mm in your total range. And without the IS, you may be disadvantaged. However that actually depends on you (e.g. whether you have shaky hands). I'm fine without IS.