Before I get into my query, let me say, I would also throw the new Tamron 90 macro into the mix, except that, focal length-wise, it's too close to my 85, so repetitious. I rented the Tamron 90 macro and was very impressed by the results. If you don't have an 85, it's worth a look.
I have a 20 year old Canon 100/2 lens. It served me well in film days, but isn't quite up to the task of hi-res digital (at least my well used copy). I verified this using FoCal with the lens on both my 1Dx and 5Ds. The results were largely "acceptable" versus excellent for most of my other lens. so, I'm on the hunt for a replacement.
I've settled on getting a macro, not that I do a lot of close-up work. It's more for the versatility, but I'd use the lens largely for full figure fashion and beauty versus true 1:1 work. I borrowed the 100 macro IS from Canon CPS a couple of times. I liked it, though not really blown away by it. I didn't find the focus all that fast nor accurate. (Of course, anything I own, I'd calibrate with FoCal, so that's not that big a deal.) I tried to find the Sigma 100 Macro OS to rent for a shoot to try that out, but haven't found anyone who rents it. (My usual go-to is Lensrentals.com.)
Has anyone use the Sigma extensively and have thoughts on its AF and accuracy, but even more, resolving power? I find the 1Dx is, needless to say, more forgiving than the 5Ds. Thanks.