Pros
There are very few companies where you have a real opportunity to change the world, to work with some of the smartest and most dedicated people on the planet, and where your customers will be proud of you, clap you on the back, and tell everyone they know to use your product - salesforce.com is one of them. Rarely does a week go by when I am not astonished by the quality of the people I work with. The company is relentlessly focused on customer success. This focus is real, and not just a marketing message. The product itself is hands-down the best business application I have ever used. The salesforce.com foundation is just plain cool and makes me proud.
Cons
No company is perfect. Salesforce.com isn't for everyone. The pace is relentless. It's a rocket-ride. But the flipside of that is that everyone is driving in the same direction and everyone knows the mission. For people who already think they know how things should be done, who understand how real companies operate, who learned their lessons in a prior decade and think they can come and bring these ideas to help salesforce.com "grow-up" - they usually feel like they are banging their heads into a wall. Everyone who joins starts by starting over - learning some of the fundamentals that make the company different, and IMHO better than the traditional software or technology companies.