Our friends and neighbors, Chris and Laura, were in dire need of door replacements. Their house was a victim of a shoddy flip and they've done a lot to bring it back to life.
All of the interior doors seemed to be made of pressed cardboard and a slight tap seemed to put a dent in them — they were terrible. The front door was solid, but very inappropriate for the house. This is where we come in... seeing as door replacement has become kind of a "thing" for us.
The front door is an exterior-grade birch door with a fetching orange paint-job and appropriate hardware. The interior doors are luan (like the originals) and will be oiled and waxed as original. We used a commercial-grade knob and lock on the front door — we even matched the original-brand hinges. All good stuff which completely changes the feel of the house.