This is the research I did before I decided to buy the 550D...
(photos and data taken from canon.de, prizes from amazon.de 06/2010)
I searched for a cam to shoot great photos and videos with, so I compared the 550D, 7D and 5D MK II. As you can see, the 550D is the cheapest of these 3 cameras. It has an APS-C sized sensor like tke 7D which is average for beginner and semi-pro DSLRs. It works with Canon EF and EF-S lenses, but you have to take care of the crop factor. The 550D has the same video modes as the 7D and an even better display. Other advantages are that this cam is smaller and lighter then the others and that it works with much cheaper SD cards (I know that this in fact isn't really an advantage (stability, speed), but for me it is because I like to have my cam with me all the time). The build quality is not as good as on the 7D or 5D MK II - but I don't shoot in rain and I don't want to drop it so I don't really care about this that much. As I said before, I'm no professional, and I don't shoot sports that much - so 4 pictures/second are okay. Same fact for the AF sensors - I only need the middle one, so this is no big disadvantage to me.
The 550D does shoot really nice pictures, but the best thing about this camera is the manual control you have when shooting videos (same as on the 7D). So if you want to have a really really good DSLR for the prize of an average one - go for the 550D. You won't regret it.