Gameboy was without a doubt my most played gaming device. EVER. Or to be more specific, the Gameboy Color.
So I think its more to do with the games on the actual device rather than device itself. Instead of trying to fit miniature versions of console games on handhelds, I think games that are very simple, addictive and fun to just pick up and play should be brought back. Who knows how many hours I have spent on Pokemon or Tetris?