In my couple of games through Keep on the Shadowfell, I've never seen the Kruthik Lair played, either as a player or a DM. That's probably because it's an unappealing dead end tacked onto the back of the Maze of Caves. There's absolutely no reason to come here, and the general atmosphere conspires to subtly hint to the players that they're going the wrong way.
I've always felt that if you want to subtly hint that the players are going the wrong way, a brick wall barring their passage is more effective than making them fight bugs for 40 minutes, but clearly the writers of Keep are of a different persuasion. At least it's a reasonably decent encounter, if anyone ever finds it.
Kruthiks come from the long tradition of D&D monster design that involves taking two unpleasant animals and jamming them together. In this case, they're "six-limbed reptiles with insectlike traits", so a kind of ant-lizard, if you will. This encounter involves the players taking on a hive of the little buggers on what is unquestionably their home turf.
Apparently the players aren't the first humanoids to come this way. Kalarel's goblins have had a go at containing the kruthiks (although I'm not sure how they got past the rats and the ochre jelly to do so). They've dug some concealed pits to try and trap the ant-lizards, but all they've succeeded in doing is making the lair even more deadly. The kruthiks avoid the traps, but the players will have to dodge them.
I've talked before about Keep on the Shadowfell being a kind of player tutorial; we've seen where the bull rush was introduced, and a variety of split-level combats establishing the principles of Athletics and Acrobatics as skills. Here we're seeing a new type of combat action - the squeeze - and a new monster ability - the aura.
As you'll see on the map, the kruthiks are initially separated from the the players by a selection of narrow tunnels. The kruthiks can move up and down these crallways without significant difficulties, but if players want to use them they'll have to squeeze through. Squeezing involves moving at half speed, granting combat advantage, and making attacks at -5; a squeeze move also provokes opportunity attacks. This is really a sucker-test for players; as soon as someone tries it, the kruthiks will block off the exits from the narrow tunnel and take turns ripping the trapped player to shreds.
The other new mechanic is the aura. Kruthiks have the ability: "Gnashing Horde: aura 1; an enemy that ends its turn in the aura takes 2 damage." This isn't a big penalty; that's less than 10% of a first-level character's hit points. In addition, the Monster Manual reveals that similar auras don't stack, so there's no particular horror in being surrounded by these things. (Pity the group playing this module as envisaged though, without the then-unpublished core rulebooks.) The relatively small penalty is a good sign this is intended more as a learning experience than as a real threat in the encounter.
Players who overcome the kruthiks will be able to loot a small pile of gold, and a potion of healing. They'll likely be left wondering what the point of the whole encounter was, which puts them in exactly the same boat as me.