I have tried all the main braun brush heads and these are my favourites. They generally last about 3 months, so although they are a little pricey to buy it still works out at around only 1.5 pence per clean. The price does move about a bit on Amazon so if you buy 2 make sure you do it when the price is less than half that of buying 4.
A word on the floss action. Although they are good these do not replace the need to floss (worse luck) - well not for my teeth anyway, so you will likely be disappointed if you have that in mind. Having said they are better than the standard brush in my opinion - and therefore worth the premium price.