UV-sterilizers conceptually can be very effective in killing unwanted organisms in the fish tank. The problems concerning their effectiveness are significant, though. Like mentioned, the "dwell time" or, the time that an organism is exposed to the UV-light inside the sterilizer must be adhered to (meaning, not exceeded) depending on what it is that you want to kill...for example, algae suspended in the water column as opposed to parasites. This is generally approached from the standpoint of gph of the pump that drives the water through the UV-sterilizer. And, as mentioned by cisco, the general rule to follow is, the slower the better in terms of kill-effectiveness. All manufacturers will state what the optimum gph you should use to move the water throught their UV-sterilizers, but lately, there's been a tendency to recommend movement much faster than it is wise, and so I'd recommend undercutting the recommended pump...even significantly. Then there is the limited life-span of the peak effectiveness of the UV-bulb. I don't know if there's a definitive way to tell how long the bulb will last, but if it's run 24/7, I would think that after 9-months or so, it's time to replace it to keep the sterilizer at optimum effectiveness.
The major shortcoming of a UV-sterilizer is that in order to kill something that it's intended to kill...like ick...the pathogen must go through the sterilizer and be exposed to the bulb's emission...at no more than the correct "dwell time." But....not all pathogens are necessarily drawn through the bulb before they have a chance to find a fish "host" and do their dirty work.
With all of the live rock and sand with all their crevices and holes and nooks and crannies, some of the pathogens are never pulled down into the sump before going into the UV-sterilizer if that's where its water-pump is located as more than likely, most are. This enables some of the pathogens, like ick, to get to a "host" safely before it's ever killed by the UV-sterilizer. For that reason, it is more effective to have the pump that brings the water directly to the UV-sterilizer in the tank itself rather than in the sump.
In any event, UV-sterilizers do offer some utility but they should never be considered anything close to a guarantee that parasites will never get a foothold on the fish in the tank.
Also keep in mind what Avi wrote above in his post. The UV can help prevent major outbreaks but will not 100% eliminate the problem. Think in terms of "keeping things in check" 
