I prefer adept because I feel that's how the game was meant to be played. There's an equal chance of me killing the enemy as there is of them killing me. I do just as much damage as they do, as it's a 1:1 scale.
I think that people should play on whatever difficulty that they have the most fun with. But whenever people say "meant to be played"--especially with a BGS game--I've got to take exception.
Do you mean the way that's it's meant to be played as designed or as implemented? Because in the real world there's always a gap, and BGS games traditionally have a significant amount of design flaws on top of that. Also, considering that the CK is one of the major features of a BGS game, I guess the game is meant to be played with mods. That really complicates it. Which mods did they mean, anyway?

Anyway, for me, adept is too easy. I literally get aggravated on Adept because I mow down foes (even at the beginning). I'm currently playing Master + PISE + Duke's Combat Tweaks, and I still feel that the game is a little too much on the easy side. I've done this with pure fighter, pure mage, pure thief, and mixed classes. It's always the same (except for a Destruction-oriented mage, which is harder). What I like about the extra difficulty introduced with the mods is that levels 1 - 10 are scary--especially with a melee fighter. I'm forced to adapt, to use poison or venom or anything I can find to get through the tougher fights. I've had a few fights where I get killed, wonder how it would even be possible to take out a tough opponent, then pull it off through better planning/movement/whatever. I love that.
One more thing: 1:1 damage is not even because of AI limitations, gear differences, etc. Some of this can be tweaked with mods, but even at the start you'll have as good or better gear than the average bandit, and this gap quickly widens. And creatures don't get any damage resistance.