It just depends on how serious you are about photography. If you are just a hobbyist and just shoot casually, and don't enlarge, etc etc., than i would stick with the 55-250IS.
If you are going to be printing, want to take photography more serious, shoot a lot in low light, have a tripod - then i would bypass the 55-250 and get the 70-200L. It is about $550 used (might be a little more with the bing.com cash back), but it one of THE BEST lens optically you can get from Canon.
I will give my own review of the 55-250. I owned this lens for about 9 months.
The first thing I noticed about this lens is that the build quality was better than i expected. Seems a step up from the kit lens for sure (MK1 mostly).
The image quality was superb for its' price. It is for sure better than the 28-105 II, and is on the same level as the 28-135IS. The 17-85IS might be a little better, but for the price that these two lenses costs, they should be avoided!
The bokeh is really nice and unexpected for this price point. Colour rendition is not extraordinary, but above acceptable. IQ, even wide open, is pretty good. This lens really SHINES in bright light; no pun intended. This lens disappoints in anything less than bright light. If you have a tripod, this will negate that a bit, but if you wanted a tripod mounted lens, you wouldn't be buying this one to begin with.
So in closing, for $200 or less (look around, ebay sells them $189 shipped BIN (buy it now), and with bing cash back, it is like $180). I sold mine, in immaculate shape (as all my gear is in) for $170.
Remember, that when you buy glass used, you usually don't take a loss, especially for L glass. It is easier to sell L glass then prosumer, as the 55-250IS doesn't fit pro bodies.