I'm assuming you would require a line of sight to the sentry for the wrangler to work.
I don't think the wrangler let's you see from the sentry's POV, so all it does is draw a vector to where the laser meets wall, and shoots that way. So what we're getting is:
A shotgun designed for revenge (demos and spies)
A pistol which controls your sentry
A wrench which is either cosmetic, a special ticket, or turns enemies into ammunition
The possibility of a guitar
The possibility of a wrench (P.D.Q.) with faster build, but no upgrading (the guitar?)
I think valve's improved a fair bit since the demo unlocks.