Earlier quoted context omitted.
Buy as much as you can afford. If you expect to be using this laptop in 4-5 years time, the OSX and applications of that era will probably only perform well on >8GB RAM.
That used to be the theory - but four years now, and I've been fine with 8 GB, more than fine. I wouldn't recommend purchasing more right now - Unless you have special requirements (Running Oracle, or some intense image management/CAD system/Development platform) - Wait for the next round of upgrades in 5 years to see if more than 8 GB is needed - I'm guessing it probably won't be even then.
Hell I don't know how the people with 4 do it! In any case it's always useful to hear what people are actually doing. Can't imagine pushing 16gb when doing some basic front end work for example.