I bought a MSR Microworks filter because it filters chemicals and pesticides, soap, urine, petrol, engine oil etc, as well as bacteria, protozoa, and just about everything else accept for water-borne viruses, which are too small for carbon filters.
It is a bit big and bulky but for me, the ability to remove chemicals and stuff like that from water is essential. The Steripens are good if you always have access to fresh, running water that is clear i.e from fast moving mountain streams or taps in garages, hotels, etc.
Other than that, they're not so good - especially because they require batteries and so therefore being electronic, are arguably not as reliable as the simple, hand opeated mechanical pumps such as those from MSR or Katadyn. You must always take water tablets in case of a malfunction or for running out of battery.
On a side note, regarding viruses - I think it is relatively rare to catch them from the water if you are careful. For example, I read that they tend to 'clump' together, normally at the bottom, along with other silt and debris, etc, so making sure you always filter from the surface of your water source will again minimise the risk.
But for third-world water systems I would definitely prefer a steripen or chemical treatment as the main cause of transmission is when basically when tap water gets mixed up with toilet water. While in most third world countries, especially in cities, the water is probably chemically treated at the source, the poor state of the drainage and sewage systems often means by time it reaches you, it could have been contaminated.
If you're going to be staying in motels, or you're backpacking through India, etc, I think UV treatment is a great idea, but for camping I'd say you'd need a decent filter and/or UV/chemical treatment to be absolutely sure.