Anybody got advice/recommendations on “Pay as you go” cell phones/services?

I have no experience with cell phones but would like something for emergencies (car trouble, injury in the woods, etc). Over in the “Do you need a cell phone to survive?” discussion, droolin recommended I look at “pay as you go” options. (http://tinyurl.com/2vwjpv)

Has anyone here used a “pay as you go” phone or service? Got any recommendations on brands, models, or service providers? Are there any “gotcha’s” I need to look out for?

Thanks.