I have personally had poor experience with Zoho and would only recommend it as a truly budget solution. Salesforce, however, is worth the price of admission in my humble opinion.
I am an independent consultant and I personally use SugarCRM. It's much easier to use than SalesForce, with most of the same functionality, and is really easy to customize. There is a free basic edition, but I have the one that runs $30/month.
I agree with Jessie; SugarCRM is much easier to use, even if the interface isn't very pretty. I think they're doing a massive overhaul, though, so it should be getting even better.
We recently did a SugarCRM product deep dive that touches on some of the overhauls to be included in the 2013 version update.
Both Sugar and Salesforce are widely used and reputable software for customer management. However, neither is specifically customized for a manufacturers rep. They both can be, but that will most likely require you to hire a consultant to get that process started with you. You might want to consider going with a CRM solution that is specifically targeted for multi-line reps. With this type of solution, you will get CRM plus multi-line sales reporting and commission tracking -- both commissions to the agency and handling of any splits with the reps you work with.
I work for a company that provides solutions for manufacturers reps, MRSware. There are a couple of other industry-specific CRM's out there as well -- which one fits your agency best will depend on a number of factors. If you would like more information, feel free to contact me.