I've just started geocaching and I use the iPhone app plus a premium membership. The iPhone has worked very well so far for several successful finds, and I'm about as rural as you can get. I haven't worked with Trimble, but I do have an app called Free GPS for entering in coordinates manually, like with puzzle caches. (I can't figure out how to do it on the Geocaching app.)
I bought the premium membership because in the last week of geocaching we've spent about 15 hours at it. An afternoon at the movies for the four of us could easily add up to $30. Paying that much for all of the hours we've spent hiking and gathering as a family in just one week is a bargain.