They are a beast! Think of a big Ruger revolver in semi auto form. When I'm shooting mine at an indoor range, people from several lanes down usually walk over to see what's doing the 'booming'. Get Underwood Ammo if you want REAL 10 mm ammo at a very reasonable price.
The G20 is a great gun. Designed from the ground up around full power 10mm loads it makes shooting them easy. Get the LWD .40 barrel for it for versatility. Actually you can get several other caliber barrels to shoot from it as well, check out their site.
tbh, kind of wish i had a 10mm still. since ive been shooting my fathers G23, i have grown more fond of glocks. absolutely LOVE the 10mm round, and a big glock thats meant to handle it? great weapon.
Some of the G21 SF guns had problems with the mag release mechanism, but not all of them.
Not sure if any of the G20 SF had any issues.
Just something to watch for.