The challenges are fairly solid if you approach them in the way they're designed, but I don't think they're resistant to cheating.
Fluffy: This one's pretty good overall. Hard to bypass unless you know the secret, makes a good early warning for nosy students.
Devil's Snare: If the attacker can levitate and shoot fire (and what self-respecting wizard can't?), this one's not much of a hazard. However, it provides a soft landing for nosy students who jump down a trap door blindly.
Flying keys: Seems pretty difficult, unless they're a nosy student who happens to be an ace Seeker. However, a more powerful wizard could easily cheat this: "Accio Silver Key."
Wizard's Chess: Seems like a good obstacle. As powerful as wizards are, getting bludgeoned by a giant stone statue will hurt. Takes an unusual skill to bypass that an attacker may not have. Giant stone statues seem resistant to brute force.
Troll: Seems a little redundant after Fluffy, but unlike Fluffy, there's no easy way for a nosy student to bypass this. Quirrel just knocked it out with brute force.
Logic Puzzle: Three words: Flame-Freezing Charm. But if Hermione is right about wizards not having much logic, I suppose this is a good trap.
Mirror of Erised: This is the big one. A thief, by their very nature, can't bypass this. There doesn't seem to be an obvious brute-force solution, either. You need a nosy student (stopped by the troll earlier), or I guess Dumbledore himself.
So basically, we've got five obstacles that can be bypassed by nosy students or brute-forced by a stronger wizard, one obstacle which can't be passed by nosy students, and one that's totally bulletproof. So my theory is the first traps are designed to be a good adventure for a group of nosy students, while the troll keeps nosy students out of the real security system. The final challenge can actually stop thieves.