Yesterday I was looking at a Chartbeat gem that accesses the Chartbeat REST API . The entire class is 40 lines of code, however it's coded so weirdly that you'd have to read the source in order to use it. Every API call was a method_missing call, so instead of doing (in irb)
puts Chartbeat.new(:apikey => 'a', :host => 'b').public_methods
The code does look magical, and kudos to the developer that wrote it for the ingenius use of method_missing, but IMHO it's a bit to magical for my tastes. I like to look at a library's documentation and instantly know what methods I'm allowed to call and what exceptions/results I'm going to get back.
 Edit: Added explanation since, on a second read, it seems like I'm digressing from the topic.