I recently purchased a filter to place on each of my lenses. Now that I'm using my camera much more frequently, I wanted to start protecting my expensive glass! Both filters I purchased are UV filters, which means that in addition to protecting my lenses from scratches and fingerprints they are also protecting them from ultraviolet light.
In the past, filters were known to create lens flares and interfere with image quality. Now filters can provide a lens with the protection it needs while the photographer hardly knows it is there! This has been the case for me. Both filters have done a great job of protecting my lenses and I tend to forget they exist.
Based on the research I have done, it is best to use filters that are multi-coated in order to eliminate glare, flare and reflections. I have not noticed any of these issues in the hundreds (thousands?) of photos I have taken since I have put the filters on, so I would say multi-coated is the way to go.
I have noticed an ever-so-slight difference in my photos that are taken in bright sunlight. The colors pop just a teeny bit more than they do minus the filter. For me, this is a positive thing since popping colors are something I happen to love!
These filters are also very reasonably priced. I purchased both for ~$50 (total).
Here are the filters that adorn my lenses:
Hoya HMC 67mm multi-coated UV filter - I use this on my Canon EF-S 17-85mm f/4-5.6 lens
Sigma DG 52mm multi-coated UV filter - I use this on my Canon EF 50mm f/1.8 II lens