I have one of the Milescraft hinge mortising jigs for the router.
It works OK (especially since the NICE one are PRICEY!!!!) however, I absolutely hated the fact that it left mounting holes in the door.
Also, when using it to retrofit new doors into existing doors, it was very hard to line up the jig on the new door so the hinge mortises perfectly matched the existing door.
My solution was to mount a strip of HDPE and use a couple of Edge clamps to keep it in place on the door. Also, after first use, the router cuts away the HDPE and leaves a nice zero clearance type feature that you can use to better align the jig to the new door.
-- You're not a real wood worker until you've been to the Emergency Room...Twice...in one year...wait a minute, this isn't right.