UV lens filters are a popular way to protect the front element of lenses from damage, but you should make sure you invest in a high-quality one unless you want to make a huge sacrifice in image quality.
The top one is a $20 Quantaray UV filter. Bottom is a ~$70 B+W MRC UV filter. This is a more extreme example, but it shows the difference between a nice filter and a crappy cheap one. Both these shots are unedited JPEGs from my Nikon D7k with a Nikkor 17-55 ƒ/2.8 @ 1.3s ISO100.
I do realize that the top pic can be easily fixed by adjusting levels, but in my opinion, it’s always better to get the best picture you can get out of your camera before editing. [#]
His last sentence is a gem: to achieve the best images, you want to make sure you’re squeezing out the best image quality you can from each step along the way.
Image credit: Photographs by EvilDoesIt and used with permission