Yeah, a good friend of mine used it two years ago on a steer. Great looking calf, but he was WAY too hard at state fair, and he was fed the optaflexx the three weeks before state fair. He had a ton of muscle from it, but I liked the steer earlier in the summer, because he looked softer, and his loin was smoother.
We've been using it heavily down here since about 05. The other replies are correct. They have to be sound enough to handle the extra muscle. It sure seems to tighten one up on the front end if they already have problems there. And they have to be very well finished before you ever start it. Every once in a while it seems to make one quit eating.
We use it in conjunction with Revalor S implants and it makes a noticeable boost in muscle in the last 60 days.
We feed the Showflexx version distributed by Sullivans. I guess its premeasured for the 42 day supply for one calf (you feed each calf one bucket). We start them on Showflexx and implants (sometimes a little earlier on implants) 60 days before terminal and when we run out, we run out. We are usually starting to pull calves down or feed them differently a week or two before terminal. So technically, I guess we are adhering to the 42 day limit. Some calves you pull off quicker if they start getting tight or if you decide you need a little more finish.