The first lens I bought was a EF-S 55-250mm as the complement to the kit lens. It's a good lens for the money but I soon realised it had limitations and within a few months I went after a 70-200mm 2.8L IS. It was about 10 times the price but doesn't take 10 times as nice photos in most situations, but that's what i had to pay for that extra bit of quality and lightning quick auto focus. No regrets - a truly superb lens!
Next lens will be something like a 24-70mm but it's not a lens I'd use a lot so it can wait until I have the cash to spare.