I've been using Nebula for a few weeks now and liking it. I've seen many talk about the unintuitive GUI and what not, personally it doesn't bother me that much, I've seen worse plugin GUIs.
What does bother me enormously is the way the faders or knobs (depending on the skin) behave. It's almost impossible to click on one and fine-tune values without causing it to jump around. This is because the hot spot seems to be very small at some random area on the fader and it changes for each skin. Whenever you click on the fader, it should not change its position, this should only happen when you click/drag it or when you click on the channel but outside the fader.
I use a MotorMix, much better than mousing around. If I didn't have one for Nebula, I'd already ran out of the house screaming and strangled someone with my bare hands! Jokes aside, you should get a midi-controller, most of them work great.