Can a person be sued if they talk bad about a company in an internet blog? (If they hate cell phone service they're getting though Sprint and they blog about how bad it is, and tell people not to get service through them....for example)

I'm not sure about the legalities, but you should be safe if you factually report your experience with the company. If you tell people not to use them, then you are treading dangerous ground, I would imagine.

been here done this.... No you can not loose such a case. They can sue you, anybody can sue you, if you are stating truth then there is nothing that you have said that shows you defaming the company.

You can be sued no matter what you do. However, in defamation you have a defense if what you say is true, or is expressed as an opinion.

Yes, and they will win if the statements are untrue.