Bear, I was reading a massive thread last night on this subject , over a hundred posts , have a look further down the gear section in OM forum.
The general concesus seemed to be that:
You should always carry one,
however the big orange bin liner is only good for sitting on and sledging, either on snow or wet gress apparently.
The baco foil noisemakers are good for nothing except making noise
And i belive there was another complaint about the type u used that centered around ripping and disintergrating after time, even if unused.
There was some call about combining both of the above for a more effective system however most praise was form the extra weight and better chance of survival from having a bivi or bothy.
Alpkit i found is doing a rather good one for £25 but i havn't invested myself yet so i dont know how good it "really" is.
If anyone does id be greatfull of your oppinion.
Most groups (Schools,Scouts outdoor centers) strongly suggest having the big orange ones but the reality is having spent a night in one its not fun and by morning there was more water inside it than outside.
I now use one as a bag linner on my big pack as its usefull as a footprint on rough ground and doubles up as a waterproof liner for my back along with the sledging thing its also can be rolled down to make a crude bowl or even water holder.
And when i do invest in a bivi, i will still keep the orange one for the reasons above but also as a big orange flag on a walking pole, to the rescue teams i hope never to meet, But big respect to you guys and girls !!
J