Is it too goody-goody of me to say that it ain't right to do this ride without paying? I mean, it's not like you're stealing from GE or Starbucks or something - it's Transportation Alternatives, and they do good stuff.
On the other hand, I've done the ride and it is annoyingly crowded. Even working your way to the front can be perilous, because slow, inexperienced riders get skittish when you go by them quickly. (I caused some lady to fall over this way, which made me feel terrible until I stopped to help her and she was really obnoxious, at which point I stopped feeling so bad.)