Pros
I've worked for this company for almost a decade. It began as just Genzyme, and was a very employee friendly, secure place to work. I've worked with the same group the whole time, and they are very smart people, good to work with. I've met other people in other groups, collaborated with some, and generally, they are intelligent and friendly. The commute can't be beat for those of us living in the MetroWest area, and the benefits given have always been decent. Since Sanofi took over, the bonus system is better.
Cons
Before Sanofi came in, the pay scale was a bit lower than at other biotech/pharma companies. That hasn't changed for the people who originated at Genzyme, but I think the Sanofi people are on a higher pay scale. Overall, pay may not be as high as other companies in the Boston area. Benefits have been reduced somewhat, and the corporate culture instituted no longer values employees, and layoffs are now routine. The company high-ups claim transparency, but that isn't really the case.