i snapped and picked up an rei minimalist bivy this last week for the hell of it. Probably better options out there but it was a last minute buy. Its farily light for a completely waterproof bivy with a bug net, and was definetly cheaper than the big dogs. Should bring the temp down with my kifaru doobie to well below freezing. Being 100% waterproof it will also be nice to be able to roll my pad and bag right up in the thing instead of having to deal with stuff sacs.
While it sounds romantic to just have a 4 season bivy and nothing else. I find that I am much more comfortable and enjoy the experience more with SilNi over my head. When the skies open up and don't stop I just don't see how you can keep your bag and gear dry and change clothes etc... in a stand alone bivy. Not to mention cooking and eating.