I suppose I should say that I've spent my entire career in what is currently called an "IP Boutique" (thirty years ago, we just called it a "small patent firm"). So, my observations of Big Firm life are entirely from the outside looking in (with no desire at all to be there).
I'd add two factors to what's already been mentioned:
1. Quality of life: This may apply more to location than type of firm, but the two are related - while not every boutique is in a smaller community, most large firms are in the major cities (NY, Chicago, LA or DC, particularly). I'll take small town life any day.
2. Variety of Work and Client Contact: Many patent practitioners in big firms spend their entire lives working for a small number of corporate clients. Because there are usually lots of other practitioners in the firm, they tend to be compartmentalized by technology. I've spoken to lots of patent people at big firms who never get to meet or speak with the actual inventors or the managers at the company - that's for partners. In contrast, patent attorneys or agents in a small firm work directly with the client right from the start, and they get to work with all sorts of technologies from an interesting set of smaller companies and individual inventors.
A Big Firm associate may go his entire career without ever seeing a meatball cooker, toilet-seat lifter or mailbox flag. I feel sorry for them.
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The first thing that comes to mind is appoint Paul brought up: the intimacy and personable atmosphere of a boutique is just lacking in a big firm. My feeling is that it is in a boutique that one can really foster relationships that will last a lifetime. Also, from a work-related standpoint, small boutique work can be much more unstable, and thus much more exciting. Thus, when a small boutique encounters a large or important case, issue, or breakthrough, it can really affect everyone in the office, and they will have to work together through the hard times and the exciting times...rather than at a large firm where the issue will likely be "assigned" to a small percentage of the company/firm, while the others hear nothing about it.
A worker in a large firm can sometimes be more of a mechanical producer, while a worker at a boutique can be more of a creative problem-solver, working together with everyone from the CEO to the intern. It's just a different spirit.
Also, large law firms can be less personable than small boutique firms. In the latter, everyone knows each other and there is a greater likelihood of camaraderie. In a large firm, it may be impossible to know everyone.
The atmosphere in a small firm can be more relaxed than in a larger firm, especially a boutique firm that is made up of large firm refugees who have made a conscious decision to run their practice differently than in the large firm environment..