I got a 5400RPM drive in a new IBM and was not that impressed. I couldn't tell any performance difference between it and the 4200RPM models I've used before - reason being, immediately after use, a laptop drive shuts down immediately (to save juice), and everytime you try to fetch something off the drive, it has to spin up again. So if you think about it, 4200RPM would actually be faster in this instance, since it takes less time to get to speed than a 5400RPM drive!
One difference I did notice is that the 5400RPM drive killed the battery much, much sooner. Took about an hour off the life of the notebook.............