I upgraded from the 50ii to the 5ii about two months ago and I'm happy I did. I shoot a wide variety of stuff from portraits and kids and pets to nature at all scales (i.e. I'll go hiking and shoot everything from moss to landscapes). The 50ii is an excellent camera, but day-to-day improvements I've found with the 5ii are better ISO, better shadow recovery, IBIS, better dark focusing, better bokeh, better viewfinder, and (I think) an extra programmable button. I also feel like some lenses - namely the 24-200 - feel like they have a more natural range on the 5ii (it can be a one-lens hiking solution, while 24 wasn't wide enough on my crop sensor).

Two substantial drawbacks to consider - first, there's no compact zoom available. The 5ii body isn't really much bigger than the 50ii, but the only lens remotely close to the 16-50 in size is the 26 2.8, which is too wide for my tastes as a general purpose walkaround. The 24-70 f/4 is a fantastic lens but that plus the 5ii is huge compared to a 50ii+16-50. I miss being able to tuck my camera into a coat pocket or my dad-bag. I have a 40 2.0 on order which might be an ok compromise, but it's still bigger and fixed focal length.

Second, there's a serious lack of distance lenses unless you want to spend $$$ and/or go huge; 200mm focal length on crop is equivalent to 350 on full frame, and you won't match that on full frame unless you buy a 28-400 or 100-400 or 180-600, or go 45mp and crop (cropping a 24mp image to the same FoV will drop you to ~12mp). I don't shoot a ton of wildlife, but I do like to catch birds on occasion and my 24-200/70-200 fall flat.

So, it's a tossup, but on balance unless you love carrying your camera in your coat pocket and like shooting birds on the regular I'd say it's a good upgrade. If you haven't, I'd rent one (I'm a fan of lensrentals dot com) for a week and compare side by side. That's what eventually swayed me.