I've used to use the Integral Designs Salathe as a stand alone for several years before switching over to a lighter bivy/tarp setup.
As a stand alone.
-It's doable, much easier with a synthetic sleeping bag than a down bag, since a wet backpack, wet clothes and everything else is going to be in the bivy with you.
-It's surprisingly warm inside the Salathe, the Todd-tex on the Salathe does a great job with wind blocking, breathability and water-proofness.
-It sucks being stuck in long storms
-Mobility is great, able to sleep just about anywhere, stealth camping is super easy.
-The Salathe is great in the fact I can "wear" it sitting up, I can cook and eat in it. This is actually great on climbing trips.
-Needing to go pee is a pain. @Eugene's question, consecutive nights of unpleasant weather is ok, fine with a synthetic sleeping bag, but with down one, nerve-wracking and not pleasant.
-Inviting wanted guests is not pleasant and having uninvited guests is even more unpleasant.
-Seam Seal will be your best friend.
-For the same amount of weight for a stand alone bivy, a tarp/breathable bivy is usually about the same weight if not lighter (my tarp/bivy set up is a 1 lb lighter than my Salathe)
If I am climbing or Big Walling, the Salathe comes, it's robust, tough, and ideal for tight quarters, for everything else, the lighter tarp/breathable bivy setup.