Save your money! A lot of the cleaning in a washing machine is by agitation, and you actually need very little detergent. Try halving the quantity and your clothes will be just as clean.
I think Which? did a report on these but I am not sure. i seem to recall they did not have much cleaning power.
I use [broken link removed] and as an added bonus, my daughters skin gets itchy using normal detergents, but not this. You can get it in a lot of supermarkets, and a shop near me even does refills, so I save on packaging (and save a few quid).
I've tried those spikey balls for drying all right (not the eco balls but just normal dryer balls) and they do seems to speed up the drying cycle. Slightly different topic, but relevant in that it saves electricity.
Ecoballs are ok for light dirt, but I think Diziet is right about reducing your load and getting the same result. Ecoballs are useless on any seriously soiled clothes.