I don't think I've ever met anyone who, after seeing all of Trigun, still didn't like it. I'm sure some people out there don't, but if you stick with it, chances are you'll find something you like about it.
When I first watched Trigun, I was watching it with a friend of mine. We both enjoyed the early episodes, but I was much more into it than he was. I'm one of those people who likes Goofy Vash just as much as Serious Vash. While my friend was enjoying the series, he wasn't as estatic about it as I was. But after we got to episode 12, we both really got into it. So, just keep watching.
However, don't expect episode 12 to be an "End all, be all, Greatest Thing Ever!" or something. I know when Bebop premired, that image was given to Ballad of Fallen Angels and some people expected too much of the episode. But ep.12 does make quite a change in the series. There's a bit of a pause after it since ep.13 is a clip show and 14 is filler, but from there on things get heavy. Basically, if you like ep.12, you'll like the rest of the series.
Anyway...so many people saying whether Bebop is better, or Trigun is better...and everyone treats Outlaw Star as an obvious 3rd place. I wonder if I'm the only one who likes Outlaw Star the best of all...