I just upgraded to 8gb in my 2.8 GHz Mid 2009 MBP. I am a laptop and memory dealer in Israel so I bought my memory in the US.
The main difference for me is being able to run a 2gb Parallels Win 7 session pretty much all of the time. Various websites (like my bank) don't work on the Mac side here in Israel so I find I'm having to have Parallels running full time. With only 4gb of RAM that was a problem.
Gadget Doctor