Yes.
UV filters do help to reduce the background levels of parasites and bacteria in the tank and also help to prevent things like green water algae, so it can help to reduce disease out breaks.
However, if you quarantine fish before adding them to your main tank, have a fully cycled tank and keep to a regular water change schedule you don't really need them.
I have one on my setup because I wanted the extra level of protection they give and have a planted tank that gets upset my meds. They're very popular on marine tanks for the same reason.
I can't really comment on good or bad units. I think go for more than you need, because parasites like Ich are actually very resistant to UV, so need a longer period in contact with the UV to kill them.