I've had my 13" MacBook since Feb '08. I don't do any moviemaking, photoshop, etc. I just use mine for Safari, Mail, iWork, Office, iTunes, Spotify, instant messenging... will probably get Aperture 3 when it's released having played around with the Aperture 2 trial... I start university in September so i'll be using it constantly for that. I also want to get into programming so will be using it for that too...
But for what I've used it for so far, i wouldn't buy a 13" again. Even just using Safari i feel the screen is cramped. And when i used the Aperture trial i felt very squashed in the window. Movies will be more enjoyable on a larger screen too.
If you've been using 15" so far, i think going down to a 13" would be a move you'd regret in the long-run. The added portability and weight will be great, no doubt (although the current 15" is only 0.5 pounds heavier than my polycarbonate 13"), but you may find yourself wanting a larger screen.
Have you played around with other people's 13" notebooks? How have you found them?
I'm very happy with my 13" for now - my first computer - but I couldn't see myself wanting to spend money on another one, except - maybe - the Air.