Sterilizing the water against pathogens keeps your discus healthy, stops disease from spreading and if breeding helps prevents fungus/bacteria affecting the eggs.
I own 6 of those units and find them very good. The only draw back is the intake sponge needs to be cleaned daily
I agree with Ben however I have a personal preference for the larger, higher capacity extrenal units (36w). They only need attention once a year when changing the tube, at the same time the quartz sleeve is given a clean (however usually not required).
You need to remember that discus can cost many hundreds of dollars, set a U.V like that will cost just over 100 bucks. Think of it as insurance. It is definately worth it.
You can get a better deal from Ben if you are in Aus. I think $120 AUD is about $100 USD anyway. You will save on postage at least.
A question for Ben, how hard is to set up this and does it make noise? How big is it? I have enough things in the tank to hide and one more big piece is not a good sight.
One makes a very minute humming noise, the other is dead silent
There is not much you can do to hide them in the tank, they're always going to be there. If that's a huge issue for you, get an in-line model.
If you get one, try and set it up at a depth that when you do water change that the powerhead can't such air through the top. That way you can leave it running all the time.
Also, regarding the daily cleaning of the intake sponge...i used to have to do this when the units were in my smaller tanks. I've temporarily got both units in my 8x2x2 and i find the sponges are still free flowing after a week now. Mind you my water is pretty heavily filtered in this tank and i'm running a fairly mid-range bio load.