I won't pretend I've spent more than 2 minutes reading up on BranchShare, but it seems to be a file-level caching system as opposed to raw TCP. In such case I'm sure it would speed up links where people commonly access the same files on web or file servers (corporate web-apps for example, or some "can in a bin" mpg which Terry from accounts put on T: drive). Doesn't sound like it works at the protocol level though and if so its benefit would be pretty specific. However something is better than nothing when it's free.
When AD finally gets some features (or I get made redundant) I'm sure I'll give it another look. You know, basic stuff like sacking getting rid of the stupid global catalogue and being a proper directory instead of an NT domain with a pleather coat, properly tuneable replication, master/slave partitioning, filtered replicas, attribute sync priority etc... Most folks have no need for stuff like that, and that's why Windows is their perfect choice, but if you want to pound the living crap out of it as a metadirectory it doesn't cut it (read: MIIS). But that's a whole different, boring, lengthy discussion