I got a single ear bud for Christmas and have been using it on my local training rides. The One Good Earbud™ puts both sound channels in one bud leaving the other ear open to traffic sounds.
Here is my review. In short, I liked it better than using only one of two ear buds. But, there are short comings.
On my next tour, I'll take both a one bud and two bud (with a microphone) headset with me.