I've got two bikes currently with internal routed cables and I've previously owned others. Internal routing is nice because it looks cleaner and the cables don't get in the way if you ever use a car-mounted bike rack to carry your bike. Also, external cable mounts tend to get paint chips that eventually allow rust to form. Many older frames with external mounts have rust around the mounts, if you ever shop for older bikes.

If your bike is maintained by a LBS, internal routing seems like a no-brainer, all things being equal. Most Italian or European steel frames seem to have come standard with internal routing over the past decade or so. However, if you are buying a new custom frame, internal routing will definitely add to the cost. I probably wouldn't be willing to pay for internal routing if buying a new custom frame because they are so expensive anyway, but I would look for that feature if buying a used or NOS steel frame.