I had 4gb up until I upgraded to ML. After 2-3 days on ML I noticed my free memory was around 300mb. I upgraded to 16gb for $65(cheap Komputerbay ram from amazon.com). An 8gb upgrade is ~45 for crucial 1.35V ram. I think its worth it.
Cool, thanks for the advice. I didn't even realize upgrading memory was so easy. I have a 2010 unibody, and Apple claims the maximum memory it supports is 8gb. Is that not really "true?"
Dig around - the 16 gig aftermarket upgrade may have been confined to some early 2011 mbp's - the 13" for sure (mine's on the way)