People (in the UK at least) regularly say “I’m hoovering” or “I’m going to buy a hoover” without thinking about the brand of Hoover. I gather the same happened in the US for Xerox and photocopying (a trend that never really made it over here). Once a word becomes common place then your branding ceases to be branding.

Once a trademark is genericized, the original owner loses trademark protection for the brand, and anyone can use it:

http://en.wikipedia.org/wiki/Generic_trademark

Things that were once trademarks: Aspirin, Adrenalin, Escalator, Heroin, Kerosene, Laundromat, Linoleum, Mimeograph, Thermos, Touch-tone, Videotape, Zipper.

The "bad" about that is that if "to google" became generic for "doing a websearch" is to stop other search enginge to start saying "Google it with Bing!".

Read the "Trademark erosion" part of the link. I posted for more info.

Take Aspirin, which was a registered trademark of Bayer for the longest time until it became a generic name. Now anyone in the US (and eleven other countries) can label their brand of acetylsalicylic acid tablets aspirin. If people are more inclined to buy aspirin, it's less likely to be Bayer brand Aspirin.