Sunless Citadel was a refreshing change after most 2e adventures and a step in the good direction. However, it didn't go far enough. The idea of a weird tree in an underground environment was sound, and so was making the adventure "just a dungeon crawl". The focus on tactical play was also commendable. However, there are problems with the dungeon design. SC plays too much like a story with a definite beginning and end - that is, it is too linear with not enough decision branches. Unlike Keep on the Borderlands or In Search of the Unknown, it doesn't do a good job of emphasizing the game's openness, where you can do almost anything in a complex environment. You go in and are pretty much expected to go along a certain path, tackle the situation and beat the "boss". That is not too bad for tournament design, but an intorductory product should probably be more open. I also have some stylistic quibbles (e.g. the portrayal of goblins/kobolds as "cute"), but this is outside good design and in the realm of personal opinion.
What would have made SC a good intro module? A little bit more extensive village and wilderness section would have helped. Instead of "the base" where you "recover" and "get quests", the village could have used more adventure hooks, a treacherous NPC guide, for instance, or a sub-quest outside the dungeon... A few wilderness encounters along the way or off the beaten path - e.g. like the infamous mad hermit or lizardman mound. Second, the dungeon should have had more alternate paths, circular routes, hidden stuff the players could find and so on. It would have made it a more rewarding experience and introduced beginning players to the concept that the decisions they make have more impact. All these changes would have pushed the module into the 48-56 page range, but with 3e's large stat blocks, this wouldn't have made it unwieldy.
Personally, I maintain that Necromancer's Crucible of Freya - some minor problems notwithstanding, like the railroading style opening - makes for a much better intro. You get a situation and you can deal with it almost any way you like.
For these reasons, I voted no. However, this is all comparative - compared to 2e products (e.g. an "intro" module like Terrible Trouble at Tragidore or Beneath the Twisted Tower) I had the misfortune of owning, it is a masterpiece. Compared to the best of the best, it is a nice try but not enough. Which is a pity because an introductory module should be among the best of the best.