Pros
If you are in engineering, Yammer is great. You will get treated exceptionally well. The rest of the company will get fed, great benefits, average pay and a pleasant environment to work along with great people. So long as you can stand tight quarters and noise. It's a great product with a lot of potential if we can get away from the "Facebook clone" image. The company is very open and most problems come to light and are addressed very quickly, usually in a fair way.
Cons
If you are not in engineering you will find your life revolving around engineers who have little concept of the real world and think they know everyone's job better than they do. If you misstep and accidentally offend one, which happens often, you will hear about it from the highest echelons of management since they won't deal with you directly. The openness of Yammer has a down side in that everyone has an opinion on everything and gets upset if a decision is made that is not "listening to them" usually this blows over quickly but can be frustrating.