I think this is perhaps the single best series of modules ever written, well, for low level anyway, Tomb of Horrors and Return to Tomb of Horrors will always hold top spots in my heart.
Last year I wanted to start an epic level game with some friends, problem was, none of them had ever played anything above level 3, they like the low level stuff, don't ask me why. So, I had them roll up mid level characters and ran them through the whole series to get them use to the idea of characters above 3rd level.
I ran them through all 4, they hated A4 the most, no player likes to be naked. As a smart group of adventurers, they took over the keep in A2 and made it their stronghold, in A1, the party mage thought he could chase down the retreating Ogre Mage with his mount spell, who just happened to be waiting around the corner with a Cone of Cold (need I say it, down one mage), in A2, the party paladin was in the court yard and, thank to a levitate spell, he few to the wall height in the court yard, he became a pin cushion.
A3 was really weird, they seemed to miss most of the good treasure, including the sword of lions, but did well against the rest of the tricks and traps, but in the final room they fell. I opted to let them fight it out rather than the put them to sleep method. They Dimension doored to the other side of the wall of force and took out the mage and the monk and almost the fighter, but the assassin just kept hiding in plain sight and disabled them to the man, oh well, assassins are mean.
Very good series, for those interested, there was a module that came out a few years ago, it tells the continuing tale of one of the slavers, Maressa from A2, it takes place when the party is around 10th level, I forget the name, perhaps someone else knows of it?