While there aren't any statistics specific to Pierce County, I can attest to at least my coworkers and my use of OneBusAway. My coworkers both use it on a regular basis as an iPhone App when they take the bus. I have been using the SMS text message option since I'm too cheap for an internet phone.
As for my experience, I have nearly all compliments for OneBusAway. I think I may have found myself more inclined to take transit now that I can tell exactly when the bus is coming. This is quite important to me since I can bike anywhere in Pierce County almost as quickly as taking the bus. One time I found that it would be quicker to walk from UW Tacoma to Tacoma Bike than it would be to wait for the next bus. While testing out OneBusAway, it was noticed that at some bus stops, the stop id number is prominently displayed which enables the quick querying of real-time arrival information. With this addition, I can now just send a text message to check what time the bus is coming instead of carrying around a schedule book.
However, as expected with new launches, a few minor issues came up. The easy issues were fixed rather quickly according to OneBusAway creator Brian Ferris. Still though, some common problems still exist that are sometimes hard to control. These include bus operators changing or not changing the bus headsign before or after they are supposed to.
Another unfortunate item is that the stop numbers are not at every single route and are noticeably absent at Transit Centers. I also have been noticing is that it appears that a really early estimated arrival time is given for some routes that have yet to begin their journey (especially my favorite 594 route in downtown Seattle). But these problems seem to be getting fixed quickly. Brian has noted that a number of small issues have been fixed already including a problem where some "Dummy Stops" were being shown as part of the bus schedule.
So that's my experience. Do any of our other readers have some feedback about OneBusAway?