Have been using it since a little before it became available to the public. Its biggest weakness is definitely knot strength, especially in the lighter tests. You can either go up in pound test to help alleviate this, use a leader, or lighten up on your normal rod power/action. I've been using a flurocarbon leader better than 95% of the time, mostly Toray Super Hard Premium-Plus High Grade Fluorocarbon in 6# tied to both 4# and 6# NanoFil (my finesse outfits). Use either a J-knot, their recommended 11-turn double Albright, or a modified Albright (Crazy Alberto) with extra wraps to link fluoro leader to NanoFil, and test knot good before using. I've been sticking with the J-knot and it's been solid.
Sensitivity and handling is great with this line, much better than normal braid IMO - way better than Fireline and quite a bit better than PowerPro. There's no "breaking in" period with this line. It's been my exclusive line for finesse jigging bass this fall, and I caught somewhere north of 800 bass since October using the same 2 spools of 4#, but primarily 6# line without a single breakoff of the NanoFil (only broke 2 off from the leader). The line will show signs of fraying after a while as some have mentioned, but it's never compromised the strength of the line from what I can tell, and I've never gone out of my way to remove long sections from the end of the line.
Haven't had any problems with the white color, other than it can be difficult to see at times. My biggest hope for 2012 is that they release this line in a hi-vis version. White shows up good on flat/dark water, but is terrible in the sun or a glare/chop. Again, using a leader and fishing primarily 2'-6' vis water around here, but haven't seen any instances where the line spooked fish.
The line is very expensive ($20 for 150 yds), but by splitting spools, I've been able to go since June on 4 outfits using just 2 spools of 150 yd. line. When you add up all the crappies and bass I've caught using this line and divide by the cost of 2 spools, it works out to just pennies per fish. The only other braid I really want to try right now is 832 by Sufix. Heard and seen good things about it and would like to compare to NanoFil.
Keep in mind this line has been expressly marketed as a spinning rod line. I still don't think they make it in over 12# test yet. I'm guessing there are some reasons behind this, one being that hooksets with a baitcaster would probably be hell on knot breakage. I also think that line digging, which hasn't been a problem with spinning for me, might become one with baitcasting. This is all purely speculation on my part, but a fair warning for those that take the line in higher tests and might want to throw it on their casting rigs. Maybe you can get away with it, maybe not. I haven't tried yet.
-T9