Is it me, or my 450D? Focussing and image quality

Another thing to consider is that the viewfinder will not show the true depth of field of an f1.8 lens. I believe that the optics of the viewfinder work out to around f5.6. So if you use a larger aperture (smaller number) then you have a hard time judging what is truly in focus. Couple that with the small viewfinder, and you can only check focus accuracy on the LCD. Higher-end DSLRs sometimes have the option of swapping out the screen in the viewfinder to allow more accurate manual focusing, but that's a big step up, budget-wise.

I have an old Nikon film SLR with a 50mm f1.8, and with that you can see how thin that slice of focus is.