I'm using the Tamiya basic putty right now to fill a couple holes on my group build project, and it seems to be working fine.  It starts to set up pretty quickly, right out of the tube.  I squeeze some on a spare bit of cardboard and the outer surface of the putty starts to set up almost immediately, so you need to work pretty quickly with it.  It thins with lacquer thinner.

In small applications, it's ready to sand within a half hour.  So far, I've been applying it and letting it set overnight just to be sure.  The instructions from Tamiya don't give any indication as to how long it needs to cure for, though.

Harder or softer than plastic?  How do I tell?  It seems to sand the same as plastic, to me at least.  Then again, i've just returned to the hobby, and my experiences with filling gaps in the far past have been pretty hilarious/horrible.  I remember building an AMT/Ertl TNG Romulan Bird of Prey, and having enormous gaps in the fit of the nose section to the dual wing section via the dual neck pieces.  I used the only gap filler I could find at the local hobby shop, Testors putty.  Hmmm... funny how nobody has brought up how that stuff performs... 

BTW, I'm gonna try the Bondo glazing putty as a filler, thanks to Grendel's latest putty evaluation vid.  Or second-to-latest, if you've uploaded your liquid putty vid already!