Personally, i don't like them, i think the look is outdated and there are better things out there. But, i've had them in the past. I wired them up to to the cigarette ligher, the most obviously 12 volt source and put the dimmer on the lower dash since you really don't mess with it.
As for the needles. Some kits have holes big enough to not have to remove the needles. But many times you have to remove. And no matter how careful, usually your tach and speedo will be off. Temp and gas are easy, just make a mark before hand with some white out and whip it off when it's right. Tach, you can either mark the idle or redline it and push the needle on. I recommend doing both to verify, whatever every you prefer, just mark idle and make sure it redlines the same.
Speedo, this is where things get interesting. The best way is to check your speed in atleast 2 gears vs the tach. If you are AT, it might be a bit harder, but you can manually shift down.
Best solution if you do need to remove the needle...Get a data logger. Then you can actually check your tach and speedo against your sensors. In my talon, it was actually more accurate afterwards (which made since in traffic). Thats what i used after my LED project, back to normal.
