We carry the S&S wrench around all the time - we don't always know until we're well into the ride (and have decided where to go) as to whether we'll need to split the front end of the bike off to hop on a train to get home (on most trains, it's much easier to hang it vertically in two parts on two single-bike hooks than it is to try to find somewhere to put it fully-assembled).
I noticed when I had the wrench in an internal pocket of a pannier bag last week that the pocket felt surprisingly heavy, which caused me to try to recall what I'd put in there to make it so, and then I remembered the S&S Wrench. There is certainly some mass to it that would be nice to minimize if anyone has a suggestion.