Here's my final take on the FC UL3, having used it quite a bit last season, and then selling it this past winter (geez where does the time go?).
The FC UL3 provides a ton of space for the weight. It's awesome in this regard. When the bugs are bad and you want space for two people to stretch out, there's no better double wall tent.
Unfortunately, it's not a great tent in the rain. It's not one specific thing, but rather the combination of smaller annoyances that make it sub-par on extended wet hikes. The main rain issues are (1) it's very difficult to not have the condensation lined sides of the fly stick to the inner, even when fully guyed out, (2), the doorway allows rain to fall directly into the tent and (3), the inner first setup means towel drying the floor if you have to set up it up during hard rain.
Regarding #1, you can stake it out well and use guylines and thus eliminate the flapping on the sides initially, but if you're camping in the rain for multiple days then the low denier fabric will eventually stretch so much that you can't get it tight, and then as soon as any condensation forms in the fly it'll be stuck to the inner.
Regarding the wind, I found in pretty good in the non-extreme winds I encountered. I would feel pretty comfortable using this tent in decent winds.
Overall, the space for the weight is awesome. For a lot of people it's a great tent. I found for me in the PNW it was too much of a pain on extended wet trips, but if you mostly hike in drier locales it's going to be great.
I'm using a TT StratoSpire2 now, which is far more pleasant to live out of in the rain. It is more finicky to setup though, so it depends on priorities.