I’ve used a Lightsabre bivy a bunch. I’ve drawn the straw to do BPL’s big bivy review to come out in the spring, so I’ve had a lot of fun since last winter testing various fabrics and bivy designs.
The Lightsabre is awfully weatherproof – at the head end. Where the fabric touches the bag, less so, but generally, I would not have a problem using the bivy as a sole shelter for summer backpacking in the Rockies or Sierras where nighttime rains are usually pretty light. Even in the worst storms, when I did get some water seeping through, it only really wet the outer shell of my sleeping bag, and the whole system was quick to dry in the summer sun.
It sheds snow even better. One would think this is a great winter bivy. However, epic crashes pretty bad at subfreezing conditions. I’m not going to get into the science behind it too much here lest I divulge all the neat stuff coming out in the articles, but suffice it to say that condensation freezing in the interstices of Epic and its breathability decreases over time through a sub-freezing night. It’s not a bad choice for subzero conditions (where emitted vapor doesn’t recondense in the pores) or for above freezing conditions (where condensation doesn’t freeze in the pores) but woe to ye that depends on it in that 15-25 degree range on a still, humid night. It can get ugly. And this isn’t unique to the Lightsabre, but to any epic bivy.