Hi,
I've been shooting with the Canon 50D and the 17-85 IS f/4-5.6 usm... results were very good.
Then, Canon 5D mark II with 24-105 L f/4... results much better.
Tested the 24-105 L f/4 on the 50D... and not to my surprise, images are much sharper.... no need to sharpen in photoshop raw converter (CS4) anymore, resulting in less post processing, better images.
So to come back to your inquiry... the 550D will and can produce nice images, but in order to get the most out of it.. think prime lenses.
Patrick.