There is enough positive things said about 40D so I won't repeat in details. In summary a very very responsive camera and a list of nice features. One thing I like very much is actually the battery life, coming from 10D this 40D lasts forever!
Here is a list of negatives I observed. May sounds like nitpicking but potential buyers should know these up front.
- AWB performance is pretty bad under incandescent light, worst to be exact compared to 1D/1Ds series and 10/20/30D. Somewhat pink/orange look rather than my 10D's yellow/orange look. The latter is relatively easier to fix in my experience. You should use gray card whenever possible.
- Shadow noise is evident starting ISO200. Review tests you can find online cannot do a comprehensive estimate on noise performance because the 40D has a great range of good/bad scenarios. If possible, exposure for the highlight to minimize the shadow noise effect. On average though, this may not always be the case and that's why we hear some complaints about 40D noise.
- Fine-detail color graduation can be a problem in some cases. Most evident in skin tones. A worst case is skin tone under shadow, the color noise is quite evident and color noise reduction in DPP may result in ugly blocks of dark colors. Lightroom seems have a smarter color noise reduction and not so "blocky". By looking at the original image you would know the root problem is the color noise, it's almost impossible for software to remove it while preserving the fine color graduation, the information is simply not there.
- Grip is better than 20/30D as I heard, but compared to my 10D I still feel the grip is a little too thin (I have relative long fingers).
- LCD is much much bigger coming from 10D. Yet this LCD has a slight bluish color cast and can't really used for checking in-focus or not. I soooo miss LCD on Canon's pocket camera!! But anyway this fact doesn't bother me.