I'm running it in Pathfinder right now & I have to say it's been one of the best games I've ever DMed. I've been using the old 2nd edition module as kind of a "cheat sheet", and I think if you're looking for a real horror based campaign it's a good reference to have.
My group has been playing this one for about three months & our next game will be our first excursion into the actual castle. They've done a ton of stuff that's not in the book, but I think that's where the best parts of any D&D game come from.
I might be in the minority here, but I think the encounters from the book are awesome. They're challenging (with a little tweaking) and there always seems to be something that will give you a disease, drain your stats, or turn you into something unfortunate if it kills you; which I've noticed makes the PCs act more carefully than if they were fighting regular D&D monsters like orcs or trolls.
Essentially, if you want to run a game with a little railroading in the beginning followed by a completely non-linear style of play, I think you might enjoy running this game. I know my players love playing it. Not that many people remember Ravenloft anymore, and personally, I love taking my favorite settings and sharing them with new players. And there aren't that many settings I like more than Ravenloft.
With a little tweaking, I give this module a solid A. Give it a try, the pdf is free on the interwebs.
P.S. If you're running it in roll20, give yourself plenty of time to get the castle maps ready. that place is HUGE.