I recently bought a Downtube Mini with the internal hub. I'm personally very happy with it, but I intend to do most of the repairs myself: the maintenance aspect seems well worth it to me. I also commute on it, and it would not be much fun to have a deraileur problem and need to hop on the subway with a big package at rush hour. That's the best part about the hub gears. It's not without its drawbacks, though: as you mentioned, cost is one. Part of that is because Sturmey-Archer is the top brand in internal hub gears, while the drivetrains on most other Downtubes have mid-to-low-quality deraileur gears. The deraileur Mini has a Shimano drivetrain and brakes, and actually costs more than the hub gear one.
The other drawbacks are efficiency (good, well-maintained deraileur gears will have less friction than a hub gear -- though poorly maintained deraileur gears will have more), inability to shift when pedaling (this isn't actually that bad -- you just need to get into the habit of coasting while you shift, but some people don't like it), and weight (they are heavier). The benefits are maintainance (as above), the fact that you're probably getting a higher quality component (as above), a more enclosed system (no deraileur to poke you while you carry it around, etc), and the ability to shift when stopped (this isn't actually that useful to me, but some people like it).