My roommate and I both have mid 2007 Macbook Pros purchased around the same time. Last year I upgraded to Leopard while my roommate stayed with Tiger. A week ago, I upgraded to Snow Leopard, and it was the smoothest and best OS upgrade I've ever had. Everything works perfectly so far, and I love the boost in speed and my faster startup and shutdown. I especially love that I freed 8 gigs on my nearly full hard drive. Then I upgraded my roommate to Snow Leopard...different story. Everything was instantly slower and sluggish for him. His Opera 10 that he loves so much wouldn't open - bouncing at double speed in the dock for about five minutes until it finally gave an error. Firefox sometimes wouldn't open. Most dmgs could not mount and would give an error. Bluetooth file transfer stopped working. Itunes was extremely slow. It was a big mess. He immediately wanted to go back to Tiger and I felt really bad for wasting his time. How could this be? We both have almost the exact same Mac. Was it because he upgraded from Tiger to SL while I went from Leopard? I was going to get my other friends upgraded, but now I'm hesitating until I find out why this happened. My roommate is so scared of OS upgrades now that he'll probably stay with Tiger until his computer dies.