I've bought Ravenloft and Demonweb, and skimmed Undermountain. If I had to choose one of the three, I'd give Ravenloft the nod. But...
General theme - I thought the editing on all three books was below average for WotC. I'm no John Cooper, and I really don't care about stat blocks... but when new monsters aren't even described (and have no pictures) ...when there are letters on battle-maps indicating monsters that aren't listed in the encounter... when there are mentions of page xx scattered throughout ...and when the scale on some maps has no mesh with the scale on other maps ...it annoys me. Plus, I've decided I really don't like the new way of handling encounters (1-2 pages per encounter). Huge space waster, and adds very little (at least for the experienced DM).
Ravenloft - Despite the problems above, this is the best of the bunch. Good theme, flexible adventure, multiple story possibilities, memorable antagonists. Not as "gothic horror" as I might like. Better than the original 1e module, but not up to the standard of the 2e campaign setting.
Demonweb - A real disappointment. Too Planescape for some people (Sigil, Dabus, Rule-of-Three, Ratatosks, etc). And yet, it has virtually nothing of what made Planescape great. Read the back cover: "Go to the Abyss. Meet Demonlords. Kill everything." That is almost the antithesis of what I believe Planescape to be (philosophy, exploration in mind-boggling realms, "big themes", complex plots, fearing demonlords - not killing them).
It's also grossly inaccurate. Out of the 70-or-so pages spend on detailing encounters, only 6-8 of those encounters will probably end in combat for a thinking party. Without giving away spoilers, all of the encounters in one chapter will only occur if the party attacks non-hostile NPCs. Most (80%) of the encounters in another chapter will only occur if the party is lacking an item that the plot assumes they have been given. To a lesser extent, the same is true of the other chapters.
Add to that the problem that the plot (and the plot-giver) is painfully obvious to an experienced and savvy player group (requiring reworking or railroading), a couple of plot-important legacy items (which I dislike), a group of massively powerful entities standing around waiting for the players to hack them up one-by-one... man, don't even get me started on the racial stats for the cambion. I'm a big Planescape fan, and a big GDQ (original 1e module) fan, and this one simply disappointed me. Your mileage may vary.
Undermountain - Other threads have covered this in detail, and I don't own it. From a casual read, it seems very plot-light for an adventure... and very detail-light for a campaign. Previous 1e/2e releases seem to have covered Undermountain in more breadth.
My Suggestion - My group really enjoyed Red Hand of Doom, despite high PC casualties. Tons of gameplay, great roleplaying and combat moments, very memorable. I also highly recommend the Dungeon adventure paths. We really enjoyed Age of Worms, and Savage Tide is proving to be a good experience as well.
