Okay hold the phone here.. ; I was going to recommend the 70-300 IS as an alternative to the DO IS. ; It's significantly cheaper AND reputed to be slightly sharper. ; But.. wait.. you've already got the 55-250? ; That's an IS, right? ; I know, you said that in the beginning, but I didn't quite process that bit of info.
I'm not sure the 70-300 is a significant enough upgrade for you. ; At least to me. ; I think it'll offer better optical performance, but I'm not sure how much more, it looks like the 55-250 may be a surprisingly decent lens for the money. ; The extra 50mm of reach won't be a tremendous difference either. ; It might improve the quality of the long end of your reach.
Let's try this. ; What do you want to use the lens for?
Oh, and I say this next bit jokingly.. beware L glass. ; If you've never used it you don't know what you're missing, and ignorance CAN be bliss. ; Once you get your hands on an L lens.. or should I say, L lens, you'll never want to go back. ; They have a satisfyingly solid feel (which, admittedly, does mean they're a bit heavier), and the zoom and focus actions are luxuriously damped. ; The 70-200F4 that Gary mentioned seems to be a popular gateway L lens. ; It's among the cheaper L lenses and if you've started out with just a 17-55 kit lens it offers a useful expansion of focal length range. ; That's how I got hooked, I was looking for something to use in the zoo and wasn't quite ready to shell out $1000 or more for the longer options. ; And I don't believe the Tamron 200-500 existed at the time.