I've been doing some research and speaking with friends across different fields and seems like there a lot of issues with ML. I'm currently on Snow Leopard, but need to upgrade for my GTX 570 graphics card, and I definitely don't want ML until I hear more positive reviews!
As you may know, apple removes the ability to purchase their old OS when a new one comes out. I came across an article which explains one way of getting Lion.
0. You will need to have downloaded 10.7 before, or at least have a friend / colleague who has done so who will sign in with their Apple ID for you.
1. Open OS X App Store. If you don't have it I guess you;re on an even older OS, you may be able to download it or failing that get a DVD copy of Snow Leopard.
2. Sign in
3. Alt+click on purchases
4. Lion should now be visible in your download history and can be downloaded again directly from here