yes,
basically whenever you need to load something from the memory card. the gallery will be much quicker. then any time you go into the SD card with a file manager you may notice a difference (it is already pretty quick IMO)
you may also notice a difference in download (to your SD, not items from the market since they are stored on internal memory) speed from the internet or whatnot.
is it worth it??? i really dont think so, but if you do a lot for downloading/file management it may be a good investment. i have no complaints with my stock class 2 card that came with my DX.