My experience with Calpine, through it's management team, has almost always been very positive. It is a company that respects and supports it's employees. Great wages and benefits. Opportunities for advancement are there for people who are willing to put in the effort and persistence. It is a company that you can have a career with.
Cons
Not very good name recognition. When I say I work for Calpine, most people make me repeat it twice and then spell it.