I used MyWi earlier today on BC Ferries, and it worked fine (3G iPhone).
However, I've found the phone sluggish and exhibiting odd behaviour (I wasn't getting voicemail, for example); everything has been fixable simply by doing a full power-cycle reboot. So I'm just doing that first thing in the morning and it seems to be less buggy.
I might do it again and turn off the wallpaper, as polaris pointed out - it just seems to kill performance.