I've tried them for the past 6 months and I think they work pretty well for what they are. AFAIK the RDA of protein is 0.8g/kg/day however if you are exercising fairly regularly there is quite a lot of research that says you should be taking more than that (if you do endurance sports ~1.3 g/kg/day and if you do ridiculous amounts of weights anything up to 2g/kg/day). It is perfectly possible to get this amount of protein in a well balanced diet but does require eating an awful lot of tuna e.t.c. (protein also comes from Eggs,Meat,Milk) which I find quite difficult and that is why I started using those powders.
However I am pretty skinny and don't do anywhere near the amount of exercise that the weightlifters endorsing these products do so I don't take anywhere near the suggested number of portions a day, I think they try and make you take loads so you have to buy more off them. I normally have 1 scoop (half a portion) first thing in the morning and then
when I go to the gym (I find it a bit of a bind to come home and eat a tin of tuna) I just have another half a portion in the half hour after finishing. I have found that if I do this I seem to recover a bit quicker and feel less tired the next day, this maybe just a placebo effect but as it works for me I don't really mind.
One thing I would say though is don't buy your powders from high street shops as they are proper expensive, have a look on the internet instead. Also don't be taken in by all the stuff they say they add to it as I'm sure that's all marketing hype, just go for the best value in terms of amount of protein (they all have different amounts). Hope this helps, been rambling for a while now,
Tim
P.s. I think quite a few people at the Bristol wall take supplements so it might be worthwhile asking them for personal experience, think some of them might sell some too but from what I remember they aren't that cheap.