I upgraded my new MacBook Pro to 8GB of RAM today ($295 from OWC, plus a rebate when I return my old RAM). It makes a huge difference when running many applications or large applications such as Aperture.
The upgrade was very easy and the Unibody MacBook Pro is very easy to open, despite having 3 screws to remove. Once the bottom is off, the RAM is exposed and can be easily replaced.
The entire process took about 15 minutes, including opening & closing the case.
I often run Xcode, Interface Builder & Photoshop CS5 at the same time, which caused the disk to thrash constantly in 4GB. Now there’s hardly any swapping and I haven’t seen a beachball. Aperture 3 was so slow it was nearly unusable in 4GB, but with 8GB it feels pretty snappy. I can move between photos seeing a beachball.