So, this weekend... out and about with the 5D2 trying to justify its purchase. Cue lots of swapping lenses, taking turns with each body on the tripod, etc etc, making comparative test shots.
To my relief, there does seem to be a noticeable improvement in quality (between my brand new, £1600, full-frame-sensored 5D2 and the crop-bodied 400D which has over 13k actuations to its name in three years of use--not exactly a stunning upset). However, of course there's more to it than just the IQ.
The first thing to notice is of course the weight. At 810g the 5D2 is about 60% heavier than the baby Rebel. It's physically bigger as well, and though my small hands were quite suited to handling the 400D, I definitely like the chunkiness of this mid-range body. Next difference is probably the most significant so far: I just lurrve the 5D2's viewfinder. Big, bright, and clear, it blows the socks off the crop-body's.
Ergonomically, it's difficult to split them. The 400D's on-off switch is in a more intuitive location for me, and of course I'm more used to its layout, but I think it'd be impossible to squeeze in all the 5D2's controls and functions while following its baby brother's design. Since I've not yet come across anything that made me go "well, that's a stupid place to put it", I'll call that one a draw.
Does the 400D beat the 5D2 at anything? Well yes, sort of. I had a collection of 4GB CF cards onto which 370 raw files from the wee camera could be squeezed. Now, with the 5D2 and its mammoth 21MP sensor, it's not so good. I'm using the RAW + JPG(L) mode, which means about 107 images, or 215 on my new 8GB CF cards. I now carry 36GB of flash storage around with me spread across six cards, but I think I could fill the lot of 'em on a single charge of the 5D2's battery. And don't forget the humungous 16-bit TIFFs I'll create in post-processing: 120MB each before I even start editing?! Feck me, suddenly my PC's 1.5TB HDD doesn't seem quite so voluminous any more...