The classes are only based on how fast the card's write speed is, not how fast it will operate to access music and pictures and other files. The only thing I can think of that would utilize the faster write speed might be the camera when taking pictures. Faster classes (6 and 10) might be useful to reduce downtime between snaping pictures.
Class 2: 16 Mbit/s (2 MB/s)
Class 4: 32 Mbit/s (4 MB/s)
Class 6: 48 Mbit/s (6 MB/s)
Class 10: 80 Mbit/s (10 MB/s)
Other than that, the only big deal is that files will write faster when transferring music/pictures from your computer. I just don't know if really matters.