I use NN, and find it very clean and pure. When I first switched to them from bargain nic base I found my 0.3% giving me head rushes and dropped to 0.24% and never looked back. Use less!
They offer different product lines, those where your batches are individually tested (and you get your own lab sheet) can take a while to get. Now, they also have pre-made batches of common items they can ship immediately.
I've found their nic as good as any and very reasonably priced (inc shipping).