Not only is it a case of do they feel comfortable, but what is the sound quality like. When you buy them ask if they have some way of trying the quality of the sound whilst still in the shop.
I got mine from the Transair shop at Victoria. The bloke their got me to plug them into a box of electrical tricks and we had a conversation through them. A bit weird in the shop, but I found that the quality of the sound differed quite a bit.
C23