How well do the beats by dre solo hd headphones work in the gym?

They could be okay depending on the atmosphere at the gym. If the gym is generally noisy and busy, then I wouldn't recommend the Solos. This is due to the noise cancellation not being too great, which means that you are likely to hear the noise of other people and other gym equipment. Also, other people would be able to hear your music if they are near you due to high leakage of the sound. On top of this, I find that they can slip off your head if you are doing vigorous manoeuvres, which isn't great if you are doing gym exercises.

However they are reasonably comfortable to wear and produce a good quality of sound to keep you immersed in your activities. They also look really stylish and slick, so you will look fashionable during your regime (which is always important).