I've had one for a while. Most of my gear is m43 but added the S5 with the 20-60 kit lens and the 50mm F1.8 to see if full frame was all it was touted to be (I'm not convinced personally).
The single AF is pretty solid, cAF is usable but is sometimes a bit annoying. I don't do enough video to give an educated opinion.
Lens selection is OK and not awfully priced for the f1.8 primes.
EVF is low res and is a weakness when you're not 100% confident in the AF.
Ergonomics are good but I've been spoilt by the Lumix G9 which is better.
The buffer is relatively shallow too, but is never an issue since I bought some cheap UHS-II cards.
When on offer with free lenses (there was an amazing offer with the 24mm F1.8 earlier this year) the S5 is really good value. The L-mount ecosystem seems relatively solid at the moment too.
I can't comment on the S5 (or any Panasonic) as I've never used one - although I have seen the deals pop up for it occasionally and I do have to admit that my interest has been tickled.
However, FWIW (from a good few years' experience), unless you absolutely need full frame in some sort of professional capacity (and even at that) I have to say it's not 'all that'.
I started out with Canon APS-C and like a lot of others just pretty much had that full frame itch that I eventually did scratch. Used it for around a couple of years then moved to Fuji (and back to APS-C obviously).
For me, unless I started shooting weddings as a professional, which I've little intention on doing, I will never go full frame again. I have garnered much more enjoyment and experience out of the fast affordable primes for Fuji than I ever did for full frame cameras and beefy lenses.
I don't mean to ramble, but if you're looking into full frame for curiosity's sake there may well be cheaper ways to do it. All that said, if it's a hobby and you are keen on branching out then have at it!
I'd pick up a G10 with phase detect too. Even if that was the only change to the G9, although I wouldn't say no to a better sensor either. The G9 handles so much better than the S5 and I love the top screen.I've held off going FF from M43 and hoping that a M43 camera similar to the Panasonic G9 is made with a phase detect AF system. If that doesn't happen I think the S5 II will keep me busy for a few years. Cracker of a camera given the features and price