Oddhermit: Quen, throwing daggers/bombs are all both overpowered and almost necessary on hard difficulty.
hulahula32: Quen is too overpowered. During my third play through for Prologue this morning I did it without using any signs or bombs and managed, only died once. And that was trying to cross under the covered bridge against the dragon.
For Chapter 1 so far, I've only been using Yrden.
Edit: This is on hard difficulty
On third play as well.
I find that now that I am really comfortable with the mechanics of the game, I don't have Quen on all the time. It is much faster to use damage spells with hit-and-run, strike and roll.
Lots of folks are quicker learners than I, in terms of combat, equipment, etc. For them, Quen was probably too powerful from the get go. It has taken me this long to feel free to use other spells more heavily.
Still, there are situations where at least the basic spell is almost a must.