In Gold, I never evolved my Totodile because I thought the sprites for Croconaw and Feraligatr looked ugly. Sure, that made my team a lot weaker, but I got through the game fine anyway, and I never intended to use him in competitive battles. I also waited to evolve my Hoothoot until level 74 or so because I liked Hoothoot better (though after I evolved it, Noctowl grew on me, and now I prefer the evolved owl). I think the reason why I evolved Hoothoot (several years after getting the game) was probably that I didn't want it to be weak like my Totodile, though.
I've frequently trained Pikachu that I haven't evolved, in all generations. I'll often train a Raichu separately, though (although at the moment in X, I actually have only put time into training Pikachu, not Raichu). With light ball, Pikachu isn't that weak (for in-game, anyway), and it's cute.
Aside from Pikachu, I tend to evolve all of my Pokémon now, even if they look better in their earlier stages. I guess I care more about their power and the competitive elements of Pokémon than I did when I was younger. That's been the case since gen III or so.