I've handled rents to own, but they seldom work out. Typically whatever the reason is that the buyer can't purchase now, is still there when you reach the point in the contract where the lease ends and the home is scheduled to close. If it's a divorce type situation where you don't want a soon to be ex on the title or you have to have 30 days of employment paystubs to show the bank for a new job that the buyer is securely in, etc, you have a chance of getting to closing, but when it's a matter of the buyer not having a high enough credit score or a large enough downpayment, typically those factors still exist later.
On the buyer side, it's a bad idea because the majority of sellers refuse to sell their homes that way so the selection is very limited. Typically the selection is only homes that have an issue that can't be overcome (on a busy street, no yard, back to a dump, etc) or the seller won't overcome (foundation issues for example), so the buyer starts out with a poor selection to chose from and it's definately not the home of their dreams.
On the seller side, while the inspections are done and negotiated prior to the buyer moving in, so you have a baseline for the condition of the home, the appraisal is done less than 30 days before closing. So if you agree to a price in 2012, but the home doesn't close for 6 months to a year, the agreed upon price is often no longer a good price. That means that if the values are going up, the seller might have been able to sell for more by the time it closes, but he's locked into the lower price. At the same time, if the values go DOWN, and the home doesn't appraise, then he is required to drop the sales price to the appraisal value because the bank won't loan the buyer more money than that to buy the home. And if the buyer can't buy, the deal is off after the seller has waited for a year to close. In addition, if it's an FHA appraisal, it stays with the home for months after it's done, so if a second buyer comes through, the seller still can't sell for more than the FHA appraisal was for. The seller has to acknowledge also that the condition that the buyer keeps the home in can affect the home's value when the appraiser comes (ie if the buyer hasn't mowed the yard recently, the home is full of trash, it appears that things aren't working, etc it can affect the value)
The seller also has the problem that if the buyer can no longer buy, but they are still in the home, they have tenant rights and getting them out can become very difficult. I've seen it go as far as getting the local sheriff to evict the tenants.
Everyone is much better off, typically, if the potential buyer rents until they are able to buy the home of their choice and they have the full housing market in their price point to look at.