Have a 30mm and 35mm one. They both are easily visible and work well. The 30mm is a little bulky and not as slim as some of the others out there but they can't really be beat for the price.
The 35mm one loosened up a time or to. In its defense it was on a 338LM but I put some blue lock tote on it and cinched it down a bit and I think it's okay now.