I have a current MBP with both 8GB and 16GB of memory. I found that with 8GB, performance is excellent. With 16GB, I normally have about 6-7GB free - Mac OS X normally uses the rest for a filesystem cache but it doesn't really affect performance that much in my experience. 16GB is going to be enough for quite a while...
Not to mention, the glaring problem people like to ignore when discussing this is the motherboard in the MBP is only going to support so much memory anyway (would not be surprised if it could only support 16gb), so talking about not being able to upgrade it to 32/64/whatever is common then is kind of moot.
