And with SBS that also means users at one site will be accessing their Exchange mailbox over the VPN which will likely be painful.
They can either access Exchange across the VPN or with Outlook's "RDP over HTTPS" Internet protocol. I't's not painful at all. Millions of people use hosted Exchange (or other remote mail servers).
For years, I used a VPN and/or RDP over HTTPS connection to my company's SBS Exchange server located in a firend's closet and that worked fine, too. Sure, if you get lots of emails with 50 MB attachments, the file transfer might take a few minutes, but for most folks, it's not a problem as long as their Internet connections are reliable.
The biggest problem working from a VPN is if you are sharing large files. Sometimes it's better to synchronize files between two servers or between the desktops and a single server.