The G-20 is one of my favorites, but not for everyone. For human threats it is really no better than a 9mm, 40, or 45 when loaded with good ammo. Where the G-20 and 10mm round shine is with hot loaded 180-200 gr ammo that is suitable for 4 legged threats.
Ammo can be a little harder to find, but is no more expensive than 45. I buy all of my practice ammo from these guys. www.georgia-arms.com
For me it is easy since they are less than 40 miles from home and sell at all local gunshows. I realise it is a little more difficult for others, but the ammo is there at reasonable prices. You just have to do a little more work to get it.
My G-20 is most often used when camping/hiking in bear country and loaded with 200 gr DoubleTap ammo that chronos 1315 fps from my gun. This is just short of 44 mag performance from short barrels and matches or beats 357 performance from 4" or shorter barrels.
Even in the back country human threats are more likely. My G-20 gives me all of the advantages of a semi-auto if human threats are the issue combined with magnum revolver perfomance for larger threats. If you travel in back country I think the G-20 or 29 is a better alternative than magnum revolvers. If you don't travel in those places you probably don't need one. Even with my G-20, I'm far more likely to leave it at home and carry either a G19 or G-26 if there is no possibility of large predators where I'm hiking or camping.
Here is a photo of my G-20 alongside my 3" S&W 629. The Glock is 3/4 lb lighter and 1" shorter. Chronograph readings show that when fired from the 3" barrel, 44 mag loads only offer a very slight edge in power over the 10mm Glock. Certainly not enough to justify giving up 10 rounds of ammo and having to lug around a heavier longer gun.