I haven't used the UMAR but I did have a K8 that functioned perfectly and was very accurate. I'm sure you already know that you'll find plenty of people with bad opinions of Olympic arms but people who actually own them are pleased.
Well, here's two reviews on the UMAR in 22-250. The first is a little long, but it's pretty thorough. The second shows a little accuracy work at 400yds and the last is some thermal action for ya. Different upper, since we have to use rimfire at night...but you get the idea.