If you want to know if a bivy will work for you, and you're looking at those mountaineering waterproof standalone bivies, do yourself a favor - get all your gear together and go car camping. Throw the tent in the car too. Lock the car and carry your backpack ten feet, get it all out, and pretend you are backpacking. Sleep out with just the bag and pad (assuming the weather is good) and imagine what that would be like with a bivy and a rainstorm lasting 20 hours. Answer the questions: where would my gear go in this storm? How am I going to get out to pee and back in without getting the sleeping bag wet? What will I do to not be bored in that time I'm holed up waiting for the storm to pass?

It's easier with the kind of bivy that is light and waterproof and requires a tarp paired up with it - but then you are starting to replicate a tent, and with very light tents on the market (the Tarptent Sublite weighs less than 25 oz, the Lightheart Solo weighs slightly more and is double wall with awnings that can roll up and turn it into just a net tent) why would you not have one when it's going to amount to the same cost?