I agree with you that it was better than most of the PG-13 horror schlock out there. I'm not much of a Hudson fan and I still think she's one boring actress, but the rest of the cast was at full speed.
I thought it rambled and repeated situations a little too often and stretched out a limited theme though. In the old days, this would have made a tight and very clever Alfred Hitchcock Hour or Thriller tele-play.
But, along with a few other plot holes that a reseeing might clarify...
(DO NOT, under any circumstances, read these spoiler questions and comments if you have any intention of seeing The Skeleton Key!!)
Warning: Spoiler! (Click to show) Spoiler
Warning: Spoiler! (Click to show) If the ghost of the black servant woman was living inside the old lady (Rowlands), then who was the black female ghost that Hudson saw in her mirror leaving her room when the old lady was nowhere around? Can the ghosts move in and out of their host bodies at will? If so, then why didn't the black male (or female) servant's ghost just leave his young host body and keep an eye on Hudson at all times?
These may be silly considerations with this kind of fare, but it just seemed there were too many ghosts roaming in and around those host bodies, inside and outside the attic rooms and so on to be so clueless about what Hudson was up to at all times.