Working with solid, established production companies are my favorite. I can wear one hat as a camera operator, and focus on doing a good job at that. They also are very aware of market rates, and generally are more concerned with just getting the right people rather than saving a few dollars in day rates, so as long as your day rate is competitive for what you do, there is very little negotiating.

Agencies suck... last time I worked with one I was in day 2 of pre-production on an agency project once when I realized they thought I was a photography company and did not actually need video... even though my entire portfolio is video...

Working directly for a client has pros and cons. Budgets are generally much tighter unfortunately, and there's usually a lot of education if they are not used to hiring out video.