Airways entry (IF you have a keypad) is a great feature, though not anywhere near as important for enroute pilot workload as the keypad itself with which entering 30 waypoints is a doddle.
I wonder where the keypad would be located? In most planes there is no obvious "FMS-type" location unless one does a lot of re-hashing.
The most important thing one needs to consider with major avionics retrofits is integration with the other kit, and the biggest chestnuts tend to be autopilot issues.
The Aspen product is a very feature-rich solution for the money but they have only just brought out the "KI-256 emulator" box so that is another piece of unproven electronics.