Just had one after a 10 mile TT, I like them, but they are expensive, so only get them when on promo, either direct from FGS, or from Home Bargains, where they have them for 99p, which works out the same as direct when on BOGOF. Way too expensive at full price from Morrisons / Sainsburys @ £1.89ish. edit, just seen they are 94p at Sainsburys at the moment, so good price.
As to if they work, having one after a hard ride, then next day I feel good, not very scientific though.