They are not expensive so I went for a pair. Last weekend I put a lot of miles on them in the Fruita/GJ area. The first thing I realized was that I needed to stop and adjust until I dialed them in just right. After that I am sold and think they are a great value. They give you another option for hand position on long rides but for me they felt extremely comfortable when I positioned them far forward and used on standing climbs. I ride single speed and stand to climb a lot. I love my bar ends but being able to bring the hands in a little and get a better grip for leverage was nice.
Well worth the cost IMO. I would say that I would not ride them through technical terrain as the inside of your thumb can bruise if you are not careful.
I have often thought of the concept on very long rides. I ordered a pair to try them out. I respond with a review but wondering if anyone else has ever tried them.