Great question. Mil-dots are very effective for ranging hard targets of know size out beyond 1500 yards or more. The problem is animals and human type targets aren't constantly the same size and won't sit still, making the range finding process even more difficult. Using given sizes for live targets and using mil-dots can be accurate out to 500 or 600 yards for me at least. Beyond that, I'm looking for a hard target to range if I can find one. But again, that's why I purchased my Lieca 1200 LRF last year.
I'm using the new 2nd Gen Mil-dot from Premier Reticle. It's a much easier reticle for ranging, but I doubt it will extend my ranges beyond what they are.
The key to any range finding reticle is practice. The more you practice with the range finding reticle, the better you'll get.