Leases are a great thing to do if the owner and leasee have a solidly written and signed contract, and understand where each other stand on the lease.
You'll find that your riding will progress more quickly when you've got your 'own' horse to ride, and leasing is a brilliant way to get introduced to the world of horse ownership. Rather than diving in and buying one, then being stuck with it if things don't work out, you can also cancel the lease if things don't work out as you thought.
I'm leasing at the moment, usually I own horses, but at the moment I am too broke to go and buy a quality youngster for dressage, so I'm leasing a horse from a local FEI dressage rider who is having trouble riding him. It's great for me, great for her. Horse is being worked and looked after, I've got something to ride and my skills are being further developed, and the owner doesn't need to worry about paying for him/looking after him etc. while not being able to ride. I've also got the option to by, at a pre-determined price.
A student of mine was a beginner/intermediate rider, but very gutsy and confident. She found a lease, the horse is lovely and quiet and it has developed her riding immensely now that she is able to ride more than once a week, and has to think for herself rather than being told what to do all the time.
Definitely a worth while thing to do - just please, make sure you have a good contract written up so no one gets bitten on the backside.