I have used the 450D for quite a few months now. I know different people have different priorities, but the plastic housing is one of the few reasons why I picked the 450D. Plastic is very light. Therefore, the 450D and the 500D are extremely light. Also, as a non-pro, I do not take my camera to extreme environments. So, the weight of the magnesium alloy housing is in fact a liability to me. Plus, the plastic housing of the 450D is very cheap to replace in case of damage (I have checked before).
Also, within the 450D, is a very capable APS-C sensor which performs well in low light.
If the plastic housing is an issue, how about the very capable Nikon D90? That's also a very good camera.
I find 450D is indeed a very solid all rounder cam, with superb low noise. Many go for higher end bodies due to the more solid body and its grip. So that's why some find 450D maybe a bit too small for their grip and also the plastic body may not be many's liking. Btw, bro, u not going for 500D, the features in this digic 4 cam is really very impressive!