I have been using the Viper - the breathable skin version - for two seasons now and cannot fault it.
It is easy to erect (oo, er, missus!) stable,
flexible, does not leak etc...An awesome piece of kit and the best bivvy I have ever had. There is a review of it in the review section here. I use the Winter Skin in winter. Three of my friends who have one - Julian Cundiff, Tim Paisley and Alan Young all swear by them.
I've read the complaints, but I don't believe them - people not following instructions I believe is the problem. I put three tiny puncture holes in mine - which was my fault entirely and it barely leaks through these in an absolute storm. I've never heard of one leaking 'naturally' - if it does simply send it back and receive a replacement.
Nashy's repair/warranty service has improved 100%.
I would not hesitate to buy one.
[This message has been edited by Paul Selman (edited 02-01-2001).]