UVLs are the industries least understood product. Consequently, there are many stories of poor performance and downright failure.
The first thing a purchaser needs to know about UVLs is that they are rate on output. There is an industry minimum output requirement of 50 micro-Watts at 36-inches away from the bulb. Unfortunately, some unscrupulous manufacturer's will tout "100 mW output" but don't tell you that it's measured at 6-inches away from the bulb.
What is a UVL designed to do. Well, it's not designed to be an 'odor neutralizer' as that's the job of the electronic air cleaner. And it is harmful to all non-UVL adjusted plastics. This includes flex duct, drain pans, EAC components, wires and any other plastics that might be in the direct line of sight of the bulb.
For these reasons, most knowledgeable installers will select a location in the return duct away from the air handler. If the UVL has sufficient output, it won't kill anything but will change the DNA of live organisms traveling in the airstream, thus robbing them of their ability to multiply and colonize.
Normally, a well educated installing company is your beset option for selection of equipment and installation. UVLs work just as well as the sunshine but only when they can reach their intended target and just like the sunshine, will do damage if installed inappropriately.
Finally, the electronic air cleaner is designed to pull out all solid particles down to approximately .3 microns in size. This includes spores, bacteria, viruses and all manner of matter of .3 microns and larger. But the EAC is ineffective for those living organisms that are smaller than .3 microns. That is where the UVL comes into play.
How do you know if it's truly doing it's job? Well, if you've got school age children or work with people who have them, you might notice that colds and flu seem to not run through your family like they used to! One person gets infected and it stops with that one person. Will you feel any healthier? Sure, when the other person is sick but not likely day-to-day. If you've got allergies, the EAC is the better solution.
Like Teddybear says, fresh air is a big help but during the pollen season, that can aggravate some allergies. Fresh air that's run through an EAC and past a UVL or multiple UVLs is by far the best solution to IAQ problems. But poor installation or sub-standard products are no help at all.
As for cost of operation and maintenance, I suggest that the cost of a new bulb every couple of years (or annually for those with only a 10,000 hour bulb) is incidental compared to the cost of allergy medications and other OTC relief strategies.