I've got two 450D bodies plus a couple of canon and sigma lenses - can't fault them. the 550d should be that much better ;-)
The only thing I'd add is that if you're using Windows, you'll need an uber computer to edit HD video footage. I'e got a quad core with 4 gig of ram, and with the latest editing software (supposedly) designed to work with the AVCHD and .mov formats, it's only just capable.