For the past 5 year I've used the Canon 350D which served me well, though had it's limitations, so I recently upgraded to the Canon 550D.
So far the 550D has performed well in the areas that lacked with the 350D, the ability to handle low light without noise as well as the increased AF is what separates these to cameras for the most part, not to mention the 18MP apart from the 8MP. The larger LCD and the new viewfinder are also key points.
The 550D performs close to the far more expensive 7D in some areas. Though 7D owners may dispute this, there are some areas where they match up fairly close. Though no doubt the 7D is a better camera.
One of the main points when deciding to upgrade was made when I saw the video quality that this camera can produce. It can record in full HD, allowing for excellent clarity when recording. Far superior to that which my dedicated video camera was getting, the ability to use regular lenses for your video recording also gives a new experience, especially with the wide angle lenses.
So far the only things to complain about is that the menu takes a little getting used to if you're used to the older EOS models, and that there is no quick deletion option for all the photos on the card from pressing 1 button like on the older models, which may or may not be a drawback for some (though having worked the menu a bit more it is still quite easy to get to in the main menu). It's more difficult to accidentally delete all your images, but it's also a bit more mission if you've got a full card and need to get a shot immediately.
