I had two blue neons under my dash until one of the connections got messed up in one of the tubes. I mouted them with a couple of small screws on the underside of the dash. There is a small groove on the bottom of my dash that they fit perfectly into. You could just barely see the tube, but they made an awesome ambient blue light. I got a ton of good comments about them. If you do it right and don't go overboard, I think it looks pretty nice.
I have 2 12" and 2 6" i had them lighting up my stero but thought i was gay cuz outside the truck it was really noticable, so now I've got the 12's under the seats shining on the floor and i havnt placed the 6's cuz i havnt found a place under the dash that they are totally hiden, yet shine good.