Avantages
Good pay, interesting and challenging work/projects, very smart coworkers to learn from, lots of opportunity for advancement if you work hard and show interest, work from home and make your own hours, don't have to deal with finding work or billing clients or any of the other grunt work that comes along with being a regular freelancer.
Inconvénients
Not very much involvement in the Drupal community - most developers don't have any contributed modules/themes and the company as a whole doesn't really encourage it. Also, good design isn't a focus as much as good development (it's a company lead mostly by developers), and lots of developers end up making design decisions on projects, leading to sites that don't look as appealing as they could.