I'm also an OM-5 owner since 2 months. I am very happy with it. I come from a Panasonic DMC-GX1, all my lenses are Panasonic but they work fine on the OM-5. The only slight disappointment was that the Panasonic lens IS indeed does not cooperate with the IBIS. Lens IS can be activated, but this will automatically suppress the IBIS, only Oly/OM lenses can join forces with the IBIS if they have IS. With the 14-42 pancake and trusty 14-140, the IBIS easily beats the lens IS. Maybe it's because those are almost as old as the GX1, perhaps the newer editions of these lenses perform better. With the 300mm Panasonic, it is unclear which of the IS systems is better, they seem comparable. I do like the fact that I can now slap even purely manual lenses on this body and still have IS.
Yes, I have used the 300mm on the GX1. It looked ridiculous and balancing or ergonomics were non-existent, like the body was some kind of odd-shaped lens cap, but I did manage to take some pretty OK pictures with it. It does seem to be sensitive to shutter shock at max zoom, the electronic shutter of the OM-5 helps a lot.
I agree that stabilisation is often unneeded, I tended to always turn it off with my GX1 in well-lit situations to save a bit of battery juice. But, I am a bit of a HDR addict and I often take EV bracketed shots to manually combine into HDR in postprocessing, and then it really helps if the frames don't move much w.r.t. each other. I have tried the OM-5 HDR mode but despite the fact that it can cover ludicrous ranges (up to ±6EV, maybe useful if I'm ever going to shoot a supernova next to a black hole or so), it enforces mechanical shutter and is rather slow. When I only need ±2EV (which suffices in most cases), plain old EV bracketing works much better, because it can use high-speed electronic shutter burst mode. Way faster and I usually only need to nudge the frames only a few pixels to align them, even with hand-held night shots.
I am doubting whether I'll sell my GX1 but I'll probably just keep it, because it's not like I'll get a huge sum for it and it certainly does not take much space. It was specifically for its small size that I bought it in the first place, as a kind of compact-on-steroids to always carry with me back in the days when smartphone cameras were still a total joke instead of reasonably decent. It is still in great condition, I only had a bit of a SNAFU with the display when I tried to clean it with an aggressive solvent, but it looks fine again with a good screen protector on it.
