I've looked into it a bit, but never found it to be right for me.
From what I understand these scenarios tend to be friendlier to the seller than the buyer. The buyer is generally responsible for upkeep, and doesn't get any of the extra money back at the end of the term if they decide to walk away.
Not to say there aren't some advantages for the buyer, you would be well informed on the house and neighborhood, and if the market improves, you have already locked in at a lower price.
I'd decide how much you are willing to pay for a straight up rental and go from there. If you don't want to buy, and he is looking to sell, you will probably have to move soon anyway.