The FK Bivy does two things really great: it protects your sleeping bag + sleeping pad from the ground, and it protects YOU from bugs. It takes no time to set up - if you're in a huge rush, and it's mild outside and not buggy, you can get away with not even using the poles. If you're going very UL, you can experiment with using a sleeping bag with a slightly cooler rating on it (a 35, instead of a 30 degree bag, in other words), as the bivy will act as an additional layer - experiment! The top of the bivy is very sheer and BREATHABLE, so condensation during the night in your sleeping bag will be kept to a minimum. This isn't an emergency bivy for alpine climbs, so if you think there's a chance you'll experience bad weather (and have no other shelter options available to you, like a lean-to, or a cave), you would want to pair this with something like the Ultimate Direction FK Tarp.
I moved to the FK Bivy from a sil nylon UL bivy, which had no poles, or any way really to keep the bivy out of my face. The sil-nylon fabric of the entire bivy made it a sure thing I woke up every day pretty wet, which degraded the warmth of my sleeping bag, and gave me the task of needing to dry it out later - ugh! Not being able to close the bivy led to mosquitoes biting my head, or the escape of heat from the opening I needed to stick my head out. Both are NOT problems of the FK Bivy. Hurray!