I have not traded there yet, but their rates seem very competitive.
At the time of writing:
1.00 BTC = 330.77457851 NBT on Changelly
1.00 BTC = 293.1618 NBT on ShapeShift
with specific amount (330.77457851):
1 BTC = 293.1626 NBT on Shapeshift
Edit:
Unlike ShapeShift, you have to sign up to use the service. But as far as I got, I was only asked for a valid email address where they emailed me my account password.
Edit 2:
They don’t seem that instant:
JUST 0.5% FEE 1.67218381 NBT
EXCHANGE RATE 1 BTC = 332.43676232 NBT
ESTIMATED ARRIVAL ≈20 minutes