The people who have .20s seem to be very vocal about promoting them. It's a different caliber but I don't feel that it's magically better that a .177 or .22.
I'd be happy to be proved wrong.
My friend had a .20 caliber Sheridan and getting pellets for it (back then) was always a problem. None of the local hardware or sporting good stores stocked them, there was next to no selection, they were a lot more expensive and delivery took a couple of weeks. Consequently, he sold the rifle. Pellets are a lot easier to get theses days.
I've always viewed the .20 as a ploy by Beeman to corner the market and sell hard to aquire pellets. No facts to support that, it's just my impression.
There's some very nice airguns made for the .20. I can see that (theoretically) being heavier the the typical .177 it would carry more energy and being lighter than a typical .22 it would shoot a bit faster/flatter. These days with heavier .177 pellets/slugs and light weight .22 ammo being available are those "advantages" still true?
In a restricted environment (like the UK) the .20 would be a desirable option. Where power limitation is not a concern I just go for the .22 caliber.
Guess that having an unusual or different rifle would appeal to some people and the .20 qualifies for that. Nothing wrong with the .20, it's just not for me.
Doesn't matter as long as we're all having fun eh?
Cheers!