Pros
The best reason to work at salesforce is because you'll get to work in a highly competitive space with some of the smartest people in software. The product is truly innovative, and is a lot of fun to sell. The compensation and perks are great as well, you will get paid fairly, get a great benefits package, and a chance to work in downtown San Francisco.
Cons
Salesforce.com moves at light speed, and is filled with overachievers. On top of these, expectations on one's performance are always high. The result of this is that you will constantly be struggling to achieve a balanced work/life ratio. Salesforce.com is fun, and it can be fun to get completely involved, and to immerse yourself in your work. However, you will find that working nights and weekends is pretty much a requirement to keep up with the pace, management expectations, and your coworkers. Unless your a workaholic, this gets old, and is probably the main reason why people end up leaving the company.