I'd advise everyone to have a go. I made a point of submitting something for each of the 6 shows in the series a year back. Two of my efforts got used.
Getting a few quid was nice but the big thing for me was having someone who knows what is needed for radio comedy saying a couple of my sketches just about scraped above the line. They did a lot of editing on one of them, but I still got the credit for the ideas.
They key thing I learned was to avoid the obvious angle on a story. Everyone will hit on that, so try something a little different. For example, I got a story though on the AV referendum using the idea of a pop chart rundown with song titles befitting how people viewed the political parties at the time (e.g Labour - Hate to Say I Told You So; Lib-Dems - Bend Me, Shape Me, Any Way You Want Me).
Also make sure the sketches are short and snappy. They normally max out at 1 minute, so don't write War and Peace.
It really makes your week getting the email through saying your idea is going to be used. It's also a useful writing credit for the future should you need it. What have you got to lose?