I have a .204 Ruger - a Ruger MKII.
My rifle has a 1-11 twist McGowen barrel, 22 inches long and same contour as the Ruger factory barrel.
It shoots clover leaf groups at 100 and comes close to duplicating .22-.250 trajectories (53 gr .224 VMax) up to 400 yds. My velocites are about 3,600 fps using 27.7 gr of IMR 8208 with a 40 gr .204 VMax. I use CCI 450 primers. I have never used any other combination so I can't make any comparisons. I have read that a 1-12 twist might not be fast enough for some 40 gr .204 bullets.
I wish the magazine box on my rifle was about .1 inch longer as I think it would load and feed better.
Performance on rodent type varmints up to 8 pounds or so is OK but not as spectacular as the .22-.250. Recoil of the .204 is light but just enough on my light weight rifle to prevent me from spotting my hits. Using a #4 or #5 or equivalent 26 inch long barrel would enable hit spotting.
I am told that barrel life of the .204 R is good because of small powder charges. I have no plans to replace my .22-250 with the .204 because I have more confidence in the .22 for coyotes.
Cheap to shoot - 7000/27.7 or over 250 rounds per pound of 8208 and $17 per 100 40 gr bullets.