I did purchase a Jambmaster and it works exactly as advertised. What an awesome tool! It is very well constructed from rigid aluminum and is light weight and easy to carry around. I estimate that it should pay for itself in labor savings in only 1 or 2 door jobs. A prehung door can be installed with precision and quality in less than 10 minutes (not including door knob or casing installation). Current pricing including shipping is about $675 or you can get it for about $500 from Homestead Doors Inc., if you order a sufficient number of interior doors from them. You will also need a router (which you probably already have), a 3/4" to 7/8" straight cutting router bit, a router guide bushing (don't forget to order the locknut along with it), possibly a router adapter base to accommodate the router guide template, a hot glue gun and hot glue sticks. The Jambmaster can also be used on exterior doors, but keep in mind that if the floor is out of level you will need to shim the threshold for a plumb installation.
The Jambmaster works on single doors and double doors from 18" to 72" wide, 80" to 96" tall and will accommodate jamb widths from 3" to 8". It also works with flat jambs, rabbetted jambs or split jambs of any thickness, as well as cased openings, bifold and bypass door jambs. I have only used the Jambmaster with well manufactured prehung doors. If you purchase poorly manufactured prehung doors you may encounter other problems such as hinge binding and improper clearances between the jamb and door. My guess is that if you care enough about quality to purchase a Jambmaster then you probably know where to buy quality doors.
So if you are a quality conscious contractor who wants your work to hold up for many years to come, but who is also concerned with speed and efficiency then this is the tool for you.