Overall performance is obviously going to be a key factor in cameras at this level, and here, rather surprisingly, the EOS 550D stumbles a little. It can switch on and take a picture almost instantly, but then so can its main rivals. In single-shot mode it has a shot-to-shot time of approximately 0.6 seconds, which isn't quite as fast as you can press the button, but is still very quick. However in continuous shooting mode it can only manage 3.7 frames a second, which is slower than either the Nikon D5000 (4fps) or the Sony A550 (5fps). I think the problem is the size of the 18MP image files. The EOS 7D, which also has an 18MP sensor, has two DIGIC 4 processors giving it an amazing 8fps shooting speed. The EOS 550D has only one processor, and it struggles to keep up. It has a buffer big enough for 34 images in high-quality JPEG mode, or six in Raw mode.
The autofocus system is, as we have come to expect from Canon, extremely good. It focuses quickly and accurately in almost any lighting conditions, including low light, but unlike some of its rivals the EOS 550D has no AF assist lamp. Instead it pulses the pop-up flash to provide low-light focusing. It will pop the flash automatically in Auto mode, but this has to be done manually in other modes.
18-megapixels is a lot of resolution by any standard, and it tempting to compare the EOS 550D with its big brother the EOS 7D, but it's not really fair to measure it against a camera that costs over twice the price. Compared to the Nikon D5000, the Sony A500 ( I haven't tested the A550 yet) and crucially the EOS 500D it obviously produces larger images, but I can't honestly say that the quality is better. There is a certain softness and lack of contrast which robs the images of some very fine detail, although in fairness that may be due to the supplied 18-55mm kit lens, which still isn't as good as the lenses supplied with either the Sony or the Nikon. It produces quite a lot of barrel distortion at wide angle, and noticeable chromatic aberration at the edges of the frame even stopped down at half zoom.
One area where the EOS 550D does score major Brownie points is in high-ISO noise control. Shots at 1600 ISO are comparable with the results from the EOS 5D MkII, arguably the best high-ISO camera on the market, and even at 3200 ISO shots are perfectly usable, although quality does break down at the 6400 ISO maximum setting.. It totally whips the Nikon D5000 in this area, and is even better than the impressive Sony A500. The question is, is this enough to justify the extra cost?
The Canon EOS 550D is a very good camera by any standard. Build quality is excellent, the control layout and handling are near perfect, high ISO noise control is superb and the new monitor is the best on the market. However it doesn't offer many more features or significantly better image quality than the EOS 500D, and is beaten in some crucial areas by cheaper rival cameras.