UV sterilizers are great! Here's a short list...
PRO's:
Will 100% kill any waterborne disease causing micro organisms and parasites (like ich).
Will 100% kill any algea in the water giving unparalleled water clarity.
CON's:
The units themselves are expensive.
Replacement bulbs can also be expensive.
That's about it. Think of it like insurance. I would never be without one on my big tank. I use a 36 watt Coralife Turbo Twist UV sterilizer. I got it after a case of ich. The ich has not returned in three years and neither has any other disease or parasite. My water has remained super clear in spite of leaving the lights on for long periods of time, heavy frequent feedings, and having lots of algea growth on tank surfaces.
Check bulb replacement cost when shopping for a UV sterilizer. It can vary greatly from brand to brand even though they may be the same wattage. It is of utmost importance to choose the right size pump to run one. Here, more volume is not desired, like in a sump. The water has to flow through the UV Sterilizer slow enough to allow the light time enough to kill the bad guys. The amount of time the water spends inside the UV is called dwell time. Manufacturers have invented gimmicks to increase this dwell time, like the turbo twist I use. Water twists around the bulb keeping it inside the sterilizer longer. Follow the guidlines on the UV sterlizer for pump sizing. Smaller(slower gph) than recommended is ok, but never bigger.
That should about cover it.