So there my gnome rogue was, in the middle of combat, trying to open a lock so we could escape from a monster we were not powerful enough to defeat. The lock was hard, but not impossible. This was a time for my character to shine! I just had to wait for the wizard to go. After that I would leave everyone in suspense as I rolled the d20. What a great moment....
And then wizard casts knock and the door opens automatically. No roll, no suspense.... just an open door.
In short, I hate the knock spell. I hate how it opens a door based on the size of it. In essence you could have the smallest, most complicated lock on the planet with DC 50, and a level 3 wizard could open it. Conversely you could have the largest door known to man, built to let Colossal Red Dragons through three abreast with a DC of 5 and a 20th Level Wizard would stare blankly at it. Knock is a horribly worded spell. I would love to see it changed in Pathfinder Beta to something where the spell caster must make a caster level check with the DC equal to the lock. This allows arcane casters a way to get around locks, but not take all the fun from a rogue that has invested skills in opening locks.