Pros: When I learned that my chipset (Early 2008 multitouch penryn macbook pro) supported 6GB RAM with the 667 speed, I wasn't sure if the investment was worth it. Now that it's installed I can say it's a dramatic change. Before I had to be careful about how many big apps I had open, and I often had RAM paging which was slow.

Cons: the only con is that Leopard 10.5.5 doesn't support 8GB of RAM at this time. Snow Leopard will unlock this capacity as the hardware can support it on the Santa Rosa chipset. The only other con is that the RAM doesn't seem to work reliably in the bottom slot- forums are showing that the 4GB chip works better in the upper (higher) slot.

Overall Review: Now I have no pagins and all of my major apps are resident in RAM. The Virtual machines in Fusion are fast fast. I feel like the floodgates have been opened, I can see that the problem with the macbook pro was not the processor speed but the RAM. 6GB hit the sweet spot for my usage. I consistently use 5GB and having the 1 extra as a buffer is perfect. Awesome upgrade w/o any reservations!