for me 8gb is plenty. used to have a 16gb ipod touch that i could fill up easily with various tv shows and movies but then realised its actualy pointless and a waste of time to watch them on the ipod/iphone (for me).
I had over 15gb of music, i went through it looking at what i had never listened to, what i hadnt listened to in ages, what i will likely never listen to and deleted them from my library, and got rid of the bad songs from some albums etc and managed to get down to about 8gb of music (not aiming for that).
Then i used some smart playlists as to sort out the music i didnt want on the iphone, like music that i would only listen to when having parties and before a night out. And one playlist that with that sort of music for the gym.
After all the sorting out was done i ended up with 1134 songs which is 6.1gb of music on my iphone. With one music video on there.
I have then installed the following apps, Facebook, spinner, shazam, auroa feint, monkey ball (truly awful btw), remote, google, crash kart.
Already have over 1000 texts on there.
About 30 photos taken on the camera, and 2 synced from itunes
With all of that and the firmware i have used 7.1GB of space with 549MB left.
So do i believe that 8GB is enough? Yes... if your the sort of person that has music to listen to and not just to say i have more than you etc and never listen to most, and if your not overly interested in watching movies on there, i stil have enough space for afew tv episodes which cud be updated with new ones automaticaly daily.
Btw i am a music enthusiast and am never not listening to music throughout the day normaly (student

) but am fairly selective with what i listen to, ish.
Hope this helps you