my 2 cents:
They're like having good lights. Which few clubs do well.
Having good, varied lights & lasers makes the whole experience more intense, and can make an atmosphere charged. Having bad/no lights can make techno seem a little sparse and dull in places.
Or maybe I've just been left a bit deflated by seeing Mills noodle about on Lost's washy soundsystem together with their "minimal" approach to lighting.
At Sonar each year the lights and the ****ing brilliant visuals grab everyone's attention and create a sense of unity amongst a franky ridiculous sized crowd of thousands of people. When they work, they work well.
Bad visuals, like bad lights, are just a distraction. Like having a tv in the pub. You want to listen to what your mates talking about, but your eyes wander onto the silent football match being played out 40 degrees above his head.
If you can do it properly, and want to spend the time and effort then fair play and good luck. Its a lot for one guy to do though. Might be worth looking for a visuals partner rather than try and do too much at once...