I'd probably give it 3.50 stars, but since I had to choose I gave it the benefit of the doubt. I mix one scoop with 250-500 ml of semi skimmed milk, so this review pertains only to that.
Flavour breakdown, ordered by personal preference:
Strawberry - My favourite by far, essentially strawberry milkshake.
Mint Chocolate - Tastes like a blended Aero, one of my favourites. Very difficult to mix, requires blender to be smooth.
Vanilla Ice Cream - Very sweet. Personally I like it but only bother if you have a sweet tooth.
Rocky Road - Much better than double rich chocolate if you want a chocolate flavour. Mixes easily compared to the rest.
Double Rich Chocolate - Okay, gets sickly quite fast. Powdery taste, will not order again. Quite difficult to mix.
Banana - Horrible. I like bananas, but there is a taste I can't quite describe, unless I got a dodgy batch I'd say don't even try it.
Edit: Since writing this I have sampled a different companies effort, BSN Syntha 6, and I will not be purchasing this again. Syntha 6 strawberry is much easier to mix and tastes better.