Recommended Small Business Firewall / UTM Device

I manage 3 small business networks.  Each has a SBS 2008 server that hosts its own Exchange server environment.  Each location has 10-12 users on a combination of Windows XP, 7 and OSX workstations.  Currently I run DD-WRT firmware on Linksys WRT54GL routers at each location.  I have Postini (Google) service for email anti-virus/spam filtering and use the free service from OpenDNS for web content filtering.  I'm running VIPRE Business Premium on the server and all desktops.  The current set-up has worked fine, but I feel like I need to upgrade to a more robust, secure firewall solution that will incorporate UTM features.  Plus I don't think you can have too much security.  I've looked at several different platforms, from Juniper, Check Point and Sonicwall to open source products like Untangle and PFSense.  Does anyone have experience with any of these products?  Two of the locations are on tight budgets, while price is not much of an issue with the third office.