I built a 204 upper for one of my AR's with a White Mountain heavy barrel...as previously noted kind of loud but shoots sub MOA groups at 100 yds with off the shelf Hornady ammo...
One of these days I hope to do some varmit hunting with it...
Just for comparison I have a Ruger M77 bench rifle in 22-250 that has a lot more recoil than the 204, it shoots extremely well but is too heavy to carry in the field....that's why I built the 204...
We shoot several .204s. It rapidly became our favorite p dog round and we shoot a lot of it. We have an AR, a Savage, and a CZ in the caliber. They will all easily shoot 1" at 100 yards. Factory ammo is a bit spendy primarily because it's all varmint ammo. I reload 32 & 40 gr V-Max and 39 Blitz Kings for it with excellent results.
My varmint hunting gun is a Ruger M77 .204 standard barrel set up w/ Zeiss 3.5-10 scope. This gun has killed 3 coyote and 1 red fox. The gun shoots the heavier 40 gr and 45 gr bullets much better than the 32 grain.
PRO - Light, fast gun that is very accurate with little recoil. Fun to shoot.
CON - Knock down power at longer range. I hit a coyote at 300 plus yards and it ran off, thought it was a good shot on my part but may have hit too far back.
This thread is giving me ideas for a new build. I watched Predator Nation the other day and one of Fred Eichler's guest hosts shot a wolf with a .204 and it dropped immediately.