Pros
If you dedicate your life to work while at Forrester, you can really build your own professional equity within the industry. You have ample opportunities to learn from others and connect to top people at Global 2000 companies. You can either be a 'lifer' at Forrester and stay and grow in the analyst industry, or after a few years at Forrester have your choice of senior/executive level opportunities at top companies that you worked closely with as an analyst. The ramp-up training is really, really good, though very intense.
Cons
Tough grind and ramp-up for people new to the analyst world. You have be able to find your rhythm quickly, doing research, consulting, presenting, and working with customer calls each day. At times it seems those responsibilities are in direct conflict with each other. They have aggressive quotas for each area, and you must course-correct quickly or they'll show you the door -- admittedly they do their best to to help you get back on track.