Is Time Capsule just a great wirelsss router and backup device
Yes, it is that.
is it any good as a basic file server / nas drive.
It is not a good NAS.. primary issue.. no internal backup. No way for the TC to back itself up.
No way for Time Machine to backup a network drive.
It still lacks any proper high speed connections for a second hard drive.. usb2 doesn't cut it.
The tools are poor.. actually almost non-existent. No partitioning.. so mixing backups and data is not a great idea.
See Q3 Pondini. http://pondini.org/TM/Time_Capsule.html
Nothing has changed with this new one.
There are no media servers. no ftp, http, or any other access method for files such as is common to the basic NAS of the world.
To say nothing of rsync and all those kinds of things.
It is still slow to spin up and quick to spin down.. fine for data.. not so great for streaming media.
Remote access can be difficult if you cannot get the TC as the main router in the network.