So I have a late 2009 15 MacBook pro with a 320 HD/4GB of RAM/ 2.53 Intel Core2 Duo. Installed Lion and after the install, it kept crashing and kernel panicking during the intro video. After several attempts of repairing the HD, it worked, yet I had no sound when on my account and the system was very slow. I was able to downgrade back to snow leopard by simply inserting my full DVD of snow Leopard and proceeded the install and had no issue. Now, I have created a small partition at least of 11 GB for Lion only, thought this time I don't have my BootCamp partition which I believe it could have affected the install since in the new partition, everything works fine no issue at all. Thinking of installing Lion on top of my main user again and hopefully it works again, but if you don't want to risk loosing your information, simply create another partition.