I am running with 4 gigs.
Speed is significantly faster than my previous MBP with 2gigs of ram.
It is hard to say what exactly makes the difference, there are many new improvements on these new ones in comparison the the 2.33ghz, 160hd, 2gigs ram I had before.
Application opening: much faster
Boot time: much faster
The entire feel seems smoother to me, but then it also felt much smoother on the last gen MBPs in comparison to mine (gen 2) so it is hard to tell.
Depending on what you have now, you may notice a significant difference or you may not. In comparison to my wife's 1.83 ghz white MB with 2 gigs of ram in it, it seems snappier, smoother, more responsive and far more capable of running taxing "tasks" i.e. Spore.