The Canon EOS is very similar to the EOS 500D and EOS 450D before it, which are themselves evolutions of a basic design which debuted in the EOS 300D, in 2003. There are a few tweaks here and there: the finish is slightly different, there are subtle styling differences (including a new mode dial), and the buttons have been redesigned for easier use. The main functional difference is that there's a new movie / live view button to the right of the viewfinder. Where it used to be (and sharing a button with the indispensable 'direct print' feature) is a 'Q' button, borrowed from the EOS 7D, which gives direct access to the Quick Control Screen. The most obvious external change - button and surface textures aside - is the new wider aspect ratio screen.
The 550D inherits the 450D and 500D's body and construction, and other than the surface finish and minor tweaks to the casing, it stays essentially the same. As such, it's primarily made from three materials; a stainless steel chassis, the mirror box which is made of high-strength 'engineering plastic' and the body made of a special lightweight polycarbonate resin with glass fiber, which also provides some electromagnetic shielding. Construction isn't bad for the price, but with the EOS 550D it is clear that you're paying for the features, not the build quality.
Canon EOS 550 vs EOS 500D: what's changed
Canon isn't officially 'replacing' the EOS 500D with the new 550D, and for the foreseeable future the EOS 550D will sit in the range between the EOS 500D and the EOS 7D (ignoring for the moment the additional confusion of the EOS 50D). The changes are partly cosmetic (a very slightly different body shell design, tweaks to the buttons and mode dial), but the real changes are under the hood.
In your hand / grip
Since the body shell is virtually identical to the EOS 500D that it replaces, the handling is (aside from slightly better buttons) also virtually identical, meaning the grip still feels a bit fiddly unless you're got very delicate hands. It's by no means terrible (and can be to an extent mitigated by use of the optional battery grip), but it's worth handling one to find out if you're someone who can't live with it. The most convenient comparison (in terms of size, weight and intended customer base) is with the Nikon D5000, which feels a little meatier.
3:2 LCD Monitor
t's always been something of an oddity of digital SLR design (with the exception of Four Thirds models) that the screen on the back is a different shape to the pictures taken, meaning images are always displayed cropped or with black bars top and bottom. The EOS 550D's 3:2 screen perfectly matches its stills (and gets closer to the 16:9 used for movies), and with over a million dots and anti-reflective design that minimizes glare, it's a joy to behold too. Very nice.
The resolution is similar to the EOS 500D (720x480 / 1040k pixels) but the wider shape makes a lot more sense. Canon has removed the air-gap between the LCD’s protective cover and the liquid crystal to reduce glare. The screen has a viewing angle of 160 degrees.