Consolidation, best way to go in my opinion. I co-built a 6,000 mailbox design in Exchange 2003 (then upgrade to 2007) that had 70+ WAN sites but only two mailbox servers (+2 cas/hub) in central site. Most sites had 1.5 to 10MB WAN max and the mailbox servers (in year 2005) were mostly board. The key is cached mode. Try hard to prevent a lot of online mode use. If you had modern hardware I would virtualize two cas/hub servers and have two physical mailbox roles with DAG's for redundancy. We likely had 5 9's uptime with that model, and this was before Exchange 2010. Server's don't have to be high end for only 600 mailboxes, but RAM on mailbox servers is key. Put as much as you can in there, like 64GB or more if you can. Use the MS RAM calculators and use that as MINIMUM. The more RAM on mailbox generally means the less disks (for I/O).
In the end you have a "beautiful" design of centralized backups, centralized admin, etc.
Cons are things like WAN outages preventing inner-site emails. Just ensure your WAN outage response time is good so people don't go days without email. If your hub is well connected to Internet the good thing is their smartphones and OWA/Outlook Anywhere could still work during WAN outage.
On bandwidth, those pipes are way more then we had for the same # of users in 2005, but it obviously matters more about the "usage profile" of your users. As a "general gut feel" without knowing more I'd say you're fine based on the average usage I've seen over the last decade.
