We have leased cows out on an 80:20 basis with the lessee putting back replacements and we took our 20% in heifers. It worked pretty well for a while and would have continued and grown except we found the wrong lessee. Expensive lesson learned.
We currently run some cows for a neighbour for a straight $ per year deal and it also works well. We rent his place and run some cows for him. We pay full rent and charge him full looking after price. It secures the rented ground for us and lets him stay in the cow business. It is no extra work to look after his few when we are doing our own. The deal is that everything is on the same health program, rations, and the cattle are not run seperately. We don't sort his calves off specifically, we just take the average of the calves. Eg: for every 10 cows he gets the average price of 5 steers and 5 heifers. We provide bulls as I don't trust anyone to pick bulls that are breeding our cows, so he actually winds up with a lot better cows over the long run.
I think a lease can work really well if you find the right partners. While you don't grow the equity in your cowherd as quick, it can cash flow a lot better than a loan that is due out over a shorter period of time, and the lease payment is 100% deductible expense, as opposed to purchase where only the interest is deducted (at least in Canada).