I have not owned a rimrock, but I have used a friend's rifle that had one on it. I honestly don't think that it was comparable to the McMillan. Most gunsmiths will also tell you that McMillan is the best as well, and also easy to work with (partly due to their excellent quality.) When you glass bed an action on an HS stock, some chipping may occur to the inside of the stock. However, that is less common on the McMillan because the HS is more porous than the McMillan.
Also, look at top quality bench rest rifles, as well as other top custom rifles - they usually wear McMillan stock, and that's for a good reason - they are the best.
I'm not saying that Rimrock is junk, but, in my opinion, just not comparable to the McMillan.
Trust me, I have no incentive to push McMillan stocks, this is just my unbiased and neutral opinion based on my own personal experience.
The only thing that I agree with that is againt McMillan is that they are generally heavier, but that's partly because they are so high quality that they are less porous and have more synthetic fibers inside of them.
Zachary