Here is my 5-part evaluation of the EOS 450D (XSi), in comparison to the EOS 400D (XTi).
Part 01: "LATEST CHANGES."
The first obvious change is the new 3" LCD screen. After spending 30 minutes playing with the XSi, I picked up the XTi and boy... does its screen suddenly feel tiny in comparison. Unfortunately, the resolution of the newer screen doesn't actually improve over the older one. They're both 320x240 pixels. I've always had a difficult time judging focus on the XTi LCD screen --- even at full magnification. The same problem persists on the XSi. With Live View, you can judge focus interactively, by watching the results on-screen as you twist the focus ring. But judging a static image remains very difficult.
The XSi now allows you to pick a color scheme for your interface menu. There's the default white screen with black fonts. There's the optional blue-ish screen, somewhat reminiscent of Nikon. The one I like best is the black screen with white fonts. More subtle, and less glare for my eyes.
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Picture of XSi and XTi
The optical viewfinder is sorta-kinda noticeably bigger. If the XTi viewfinder made me squint, the new one at least feels comfortable. The change in magnification helps (0.80x to 0.87), and I suspect the shorter eye point makes it feel even bigger. It's still not good enough for manual focus, but at least it's more comfortable now than it was before. Oh, and there's now a 4% Spot Meter in the viewfinder. Nice.
Compared to the XTi, the new XSi is noticeably quieter when you press the shutter button. The XTi was already as quiet as any dSLR in the market. The XSi is even better. The actual shutter sound is now at a lower pitch than before. But the biggest improvement comes in the sound of the mirror slap. More subdued, more muffled. It's low key, not too loud and high pitched like the XTi's. It doesn't even sound like slapping of mechanical parts. It sounds like a damped rotor, though I don't know for a fact if it's motorized or not. I know it's much more quiet. It's certainly less attention grabbing than the XTi.
A 2-second timer delay for the shutter is now available, without the need for the MLU workaround. The MyMenu interface lets you move your most-used functions (ie Mirror Lockup, etc.) to the top level menu. These are available at the press of a single button. And you can now use the pop-up flash as an AF Assist Strobe without being forced to use flash during exposure.