Hi there!
I use a harness with Sophie (who is a dream traveller and goes to sleep within 5 mins of the engine starting!!)
However, I find it a real pain in the backside as if I leave the car say to put petrol in, Sophie will attempt to follow me from inside the car and gets tangled up in the harness bit that is attached to the seatbelt! I then have to disconnect it and re-attach it back on again once she is untangled!!
I also have to keep her in the front seat as if I put her in the back she moves to the middle of the backseat to see what is happening - then gets tangled in the seatbelt again!
I suppose it could be way worse if she was a bad traveller too though!
I was considering getting a grille for the "boot", but have similar feelings towards them as yourself - I feel they are ok in estate cars, but I don't fancy putting Sophie in there in my car which is only a Peugeot 206.
Sorry - not much positive feedback for "grilles" so far!!