I've been to zero a few times with the Winter Yeti, no problems. I'm fairly certain I could go to at least -15F to -20F with that thing, but haven't had the opportunity to verify that yet. My biggest problem with the Winter Yeti is it, at times, is too warm. I'm still learning how to get a good vent with it, but I'll figure it out. I've camped in about every condition I can find (wind, sleet, snow, etc.) and it has done marvelously so far. I've even gone so far as setting the tarp up high off the ground to let the cold and wind in on purpose (testing is fun!), good results each time.
3 Season Yeti is pending, but I think it will be good down to the 20s based on the way the Winter model has preformed. I'm hoping it will give me a broader temp range and I'll be able to use the Winter Yeti on those really cold trips. Still just using the GG pad under the legs.