I used to use mine everyday last year. Three of my friends in gym had pokewalkers, so after we had to run on Fridays, we'd connect. I'm just starting to get back into carrying mine around, but I hate how you only level up one level maximum, no matter how many steps you take.
I have never used mine and probably won't ever use it. Funny since, I was super excited about HG/SS having a Pokewalker but I just never really bothered with it since I missed the Yellow Forest event, and that was the course I wanted the most
I use mine when going for a walk in the afternoon or jogging. It motivates me to excersise, because I want to discover new routes and catch new Pokémon. It's a great way to level happiness up. (Eevee-Umbreon)
I used to use it all the time, but then after about two months, I stopped using it. It lost its faddish charm. There wasn't much point to it, as it didn't offer anything that you couldn't get by trading or from previous games, really.