I'm on the fence about the 500GB model - Amazon qualify for the cash-back offer, and it'd save me £18. All in, the drive would cost around £160. Is the performance really THAT much better in real life? I don't consider even my stock 5400rpm 500GB drive 'slow' - once it's booted up, it's never slowed me down. I'm aware that the SSD has around 5x the read and write performance of my 100MBps HDD, but it's never felt slow.
Also, I'm probably upgrading my Feb 2011 model in two years. So it's like an £80/year upgrade, and that's without adding another 4GB RAM in to it as well, which is still expensive compared to last year's prices.