With all due respect to the answers here (from very good, knowledgeable people on Trulia), I disagree.
First, what you'd be paying is not a downpayment. It's an option fee: The right to buy the property at some point if you choose. That fee might be credited to the downpayment or to the purchase price...if you choose to buy. You generally lose it if you don't buy.
Second, it might or might not be large. I've done lease-options with no money. Really. More often, the amount is in the range of 2%-5% of the purchase price of the home. Any more than that, and you're risking too much money. Besides, you probably have enough for a conventional downpayment. If you're within about 6 months of sufficiently cleaning up your credit, it makes more sense to buy conventionally.
Bill is correct (and you can see it in the recommendations of the others) that agents generally don't like lease-options. It's a lot riskier for the seller who really wants to sell. Only about 30%-50% of all lease-options ever result in sales. (The numbers are difficult to come up with, but I've never heard anyone suggest it's above 50%.) And from an agent's financial standpoint, they often don't get paid until the sale occurs. So--understandably--if you tell an agent: "There's only a 30% chance you'll get a commission from this transaction. And if you do, it may be 3-5 years," most won't be happy.
So why would a seller agree to a lease-option, especially with little down? Lots of reasons. They were thinking about renting, but now might be able to get even more in monthly income plus some cash up front. Or they'd really like to sell, but can't find anyone to buy. (That worked better a few years ago, but still applies in some areas to some properties.) They want to keep renting the property, though they're open to selling, but they're tired of plain renters not taking care of the property. And there are plenty of other reasons, too. I wrote a blog on how to find lease-options; some of it explains the situations in which sellers are willing to consider lease-options. See http://bit.ly/findaleaseoption
Hope that helps.