A performance boost over the 2.5 GB, which I have built before, I could tell. The start time of applications and the start-up of the laptop itself are shorter. Now have more applications in space, you also start faster if they were ever opened. Probably does not hurt the 4GB, once running Lion. Currently I have 2.74 GB free memory after booting and launching Safari.
So who is likely to upgrade his MacBook, I can only recommend this store. On the homepage of the manufacturer there is a small tool (Mac and Win), which checks which memory is installed in the computer and how to "fit" the best computer can ... thumbs up!