My personal experience : I had 4GB of RAM and got another 4GB free for fixing a friend's PC. I never witnessed any problem with 4GB and I didn't see any improvement with 8GB. I think that sums it up quite well.
My personal experience: I had 2GB of RAM and got another 2GB because I wanted to. I witnessed problems with 2GB and saw and improvement with 4GB. Conclusion: 4GB is a good level based on just our experiences and our needs. However, the beauty of a personal computer is its vast uses in a person's every day life. Each will have different requirements and each person should address those requirements on an individual basis. 1GB on a netbook with XP may work for one person while 12GB on a heavily tasked workstation may be just enough for another. It would be foolish for one to say that 4GB is the be all and end all level of RAM for all users just because it worked for knuck and I, yet that's what some people seem to be doing with their own experiences.
This Also, isn't there also the argument that RAM is meant to be used. So if you have 4 gigs but only use 2, that doesn't mean you can't make use of the other 2. I'd rather have more memory than I normally use and options to use the rest if/how I want (or future proofing if that term isn't silly these days).
