Nick I've got the oberon mirrors on my S4Rs
I bought them direct from Oberon's UK site, here's a link http://www.oberon-performance.co.uk
I went for the 14mm fitment for my bike but Oberon recommended an 8mm bolt fitment so I guess it depends on the model of monster. The build quality of the alloy part is fantastic and the mirror is ok. Setting up the mirror is a two person job though; since you have to loosen of alien bolts to adjust the mirror off the bike then get on to see the result.
They’ve not been on the bike long say around 50 miles but here’s my thoughts.
Once they are adjusted they work quite well not as good as the originals obviously, but they allow you to see behind in both lanes for a good distance. Although not sure if there is more of a blind spot when changing lanes jury is still out on that one. The mirrors themselves magnify objects so that takes a bit of getting used to.
In short I like them, a lot cheaper than say Rizoma and look better.
Hope this helps Alan