The viewfinder was actually not that different from the 450D, I found the brightness and coverage relatively similar.
I would like to corroborate that view. I had use of both a 40D and a 450D for two days, so I could actually hold both up to my eyes taking turns looking first thru one then the other viewfinder. I was more than prepared to find quite a difference. But whether peeking into bright sunshine or a dimly lit room, I could not for the life of me make out any difference at all.
The 40D is quite a lot heavier - although trying one with the featherweight 18-55mm IS and the other with the 17-85mm IS USM is hardly fair. For the fun of it, I tried them in all combinations: The 40D with the cheaper lens is roughly the same weight as the 450D with the mid-grade lens. With this latter lens the difference between the two cams is almost negligable. I. e. if you plan to upgrade the kit lens anyway, don't be worried about the added 270g of the 40D body. If on the other hand you want rather an inconspicuous set-up for street-shooting, maybe with the 35mm/f2, which you can shlep for hours on end, it's going to make quite a difference.
Like Shonen I dislike the ergonomics of the 40D, although after I sent back the 450D and spent some more days with its larger sibling I did get used to it. The large wheel on the back, which gives you direct access to exposure comp is all good though. The joystick is superfluous, its few functions could easily be delegated to one of the two wheels. Why people are so eager on the tiny top display, when they could have the information on 3 glorious inches on the back escapes me. As it is that display lies barren between shots and I for one would need to put on reading glasses for the top deck.
The 3 custom modes on the dial are a godsent. Imagine you are street-shooting and there's this limo going by rather quick and you want a a shot of the woman in the back-seat. But you were concentrating on children in a playground in broad daylight with tree shadows - that's how you are currently set up. You could at the flick of the wrist change to Auto WB, AF servo, auto AF field plus ISO 800 for a "from the hip shot" that would've been lost had you tried to change all the parameters manually. The 450D doesn't do this. The E-520 is afaik the only entry DSLR to have custom modes.