Canon's EOS 40D camera was an amazing success when it was first released back in 2007. The 40D, along with the EOS 1D Mark 3, it was one of the first DSLR's to feature 14-bit raw files. Previously, all DSLR's cameras were 12-bit raw files .. although some still dispute whether or not 14-bit files actually make a difference, I can say from my experience that 14-bit files are better!
The 40D produces 10.1 megapixel files, which is more than enough for most photography tasks. But, if you seek super detailed 30x40 prints of landscapes, then the 10 megapixels will probably not satisfy your image quality hunger. I've gotten away with 20x30 prints, but it's a stretch and the photo had to be perfect in the first place to get a quality 20x30 print.
As a backup camera to a 5D or 5D Mark 2, it makes a perfect addition to your camera bag. Plus, they're super cheap now since most people are upgrading to 50D's and up.
On a side note, if you plan on sticking with APS-C sized sensors (the 40D has one), then I completely recommend getting the EF-S 10-22mm wide angle lens. It's an amazing lens that I would qualify as "L" quality glass .. just don't drop it 'cause it WILL break!