I wouldn't rely on Lion (or any new OS) until it's been through its first few updates. There'll be tons of imperfections in the new OS, and any of your applications might have major compatibility problems that might take them a long while to resolve, if they can at all. I used a programme (Timeline 3D) that a Snow Leopard update completely broke for my generation Macbook. All the programmers could say was 'the fault is with the OS. We can only wait until Apple updates Snow Leopard.' The next update fixed the problem, but introduced a new, serious, problem.
I say don't wait for Lion unless you're happy to wait until well after its release. If you want to buy this year, buy now and enjoy speed and stability.