Utraviolet water purification works great for some types of contaminants! These types of filter usually have to be supplemented with a pre-filter that uses another technology, usually carbon/ceramic filtration.
Ultraviolet is cost effective, and also efficient.
It's only real problem is that it can do nothing for non-living contaminants.
If your water has only living contamination--bacteria, viruses, or microorganisms--then UV might be an excellent choice for you.
UV works by essentially killing the organisms that are infecting the water.
It does this by disrupting their DNA.
For living organisms, UV is very effective.
Unfortunately, this remarkable water purification technology has two faults.
One: it may not be able to kill microorganisms in cyst form, and Two: it does absolutely nothing for non-living contamination.
These two issues are the achilles heal of utraviolet water purification.
More and more, we're finding a class of microorganism in our water that exists in dormant, cyst state when it's outside our bodies.
Giardia and cryptosporidium are two examples.
Both of these can cause violent cases of intestinal distress.
If you've ever had a bout of giardia, then you know! (I have!) One of the problems with UV is it's not completely effective against cysts, because of their hard outer shell.
This problem, however, can be addressed by having the water go through another type of filter before it goes through the UV part.
That way the cysts get filtered out.
A more important problem with utraviolet water purification is that it can do nothing to protect you from inorganic impurities, like lead, asbestos, or prescription drugs.
The reason is the UV light isn't actually filtering out anything--it's disinfecting! Just as in the case of the cysts, in order to filter out other impurities, you're going to have to combine UV with some other types of filtering technology.
In other words, you could almost make a case for not buying a UV filter, because you're going to have to use something like a carbon/ceramic filter anyway.
Although carbon/ceramic filters don't actually kill anything directly, they can filter out everything that UV can kill.
You also have the added benefit of knowing that the dead bodies of all the billions of bacteria and viruses that UV kills aren't still floating around in your water.
If you're on public water, then I can almost guarantee you that you have more issues with your water than just living organisms.
You probably have pesticides, prescription drugs, lead, all sorts of things.
An utraviolet water purification system does not actually make that much sense in this case, because you have so much other stuff to filter out.
If you're on well water, then you might actually have only living contaminants.
But still, personally I would want to know that my water is as clean and pure as possible.
There are other types of water filters that can do all of this for you, carbon/ceramic filters being some of the best.