This is just a quick bit in to make mention of some of the spectacularly undercosted HQ and character units in the Dark Eldar codex.
Dark Eldar are a fine book, but it take a bit sometimes to appreciate just how amazingly price efficient some of the units are. In particular, the named characters are some of the most impressively undercosted additions in any codex since Eldrad.
Let's start with the best: Baron Sathonyx. Baron Sathonyx makes Hellions a troops choice, and then makes Hellions a game-winning super unit, and THEN improves your chances of selecting first turn to almost deific proportions. With an army as focused on the alpha-strike as Dark Eldar, this is a huge deal indeed. All for the cost of a stock Ravager. The only two downsides: no combat drugs (so he can potentially pull his unit down on a roll of 1) and no power weapon, though S6 is more than enough to add 1-2 extra wounds on your average unit of space marines. Then you get a host of special wargear ang rules, from a 2++ to hit and run rerolls all over the place to BS, Initiative and WS that make your average Marine Chapter Master looks like a doofus.
Adrusabael Vect is a character that makes point-conscious players howl, but when we look at what he does his points are more than justified. A solid way to look at it is thusly: 100 points for the most ridiculous pre-game power ever created plus 140 to make your character capable of killing, well, everything. I've had this guy wipe out units on his own in one turn, leaving his tagalong followers wondering what to do. On one occasion, he held off no fewer than three full harlequin units with the charge wiping each out to the man. Of course, there was that time he was killed by a guardsman with a powerfist. That player earned himself an Incubi head with that one. But those instances are rare so long as you aren't throwing him rashly into a fight he can't wi... ha ha, just kidding. he'll win almost every fight except Orks 50-man guard, and maybe 20 Genestealers on his own. Seriously, the guys a monster.
Malys I don't have too much experience with, but a quick consultation of the wargear chart shows that she too is at least slightly undercosted. Malys is a Archon with a Djin Blade and a 4+ save (which I value here at about 20 points) bringing her to a solid 100 points, where she pays a further 15 for immunity to Psychic powers (not too impressive) and 15 for Eldrad's redeploy power (HOLY S--T).
Duke Sliscus is the last one on this list. 150 points is actually slightly overcosted in several key ways, but if you have more than 3 Venoms, Raiders or Ravagers in your list you have probably made his points back. Like Baron Sathonyx, this character will boost individual units to game-winning proportions (either through his improved poisons or the simple expediency of improving the combat drugs roll). On top of that, he's a solid combatant in his own right with 2+ wounds and 5+ rending (sans the extra d3 against armor). The gree retro-rockets to everything in the army typically doesn't improve much, but there are a small handful of occasions where it might be effective. Combat drugs are this guys main benefit, making units like Wyches, Bloodbrides and Hellions significnat;y better in each engagement.