Time for the odd ones, the Skitarii elites. These are the Sicarian Ruststalkers and Infiltrators respectively. They are both CC units, and both are different enough to warrant their use. The rustalkers are little more damage output focused, while the infiltrators a bit more sneaky. The ruststalkers will be harder to use, but I do see them as useful in the skitarii army.
First up lets do the infiltrators. Sats wise they are WS & BS 4 S4 T3 2Ws I4 2 A Ld8, with the princeps having an additional W A and Ld. They have sicarian battle plate, which is a 4+ and 6++, a stub carbine and power sword. The stub carbine is 18" S4 AP- Assault 3. They are bulky, have FnP, infiltrate, stealth, dunestrider, doctrina imperatives, and lastly neurostatic aura, which reduces opponents WS BS I & Ld by 1 if within 6" of the squad. This is a nice little package. They come with 5 for 185pts and can add up to 5 more at 35pts a model. The entire squad can replace their standard weapons with flechette blasters and taser goads for free. Flechette blasters are 5 shot pistols, 12" S2 shred. Not amazing but shred helps them out here, though rending would make these an actually good gun, but still not bad. The princeps can take an infoslave skull for 10, which gives 1 Ld and acute senses. Not great, as these are better just infiltrating.
So how to take them? Of the two elites, they are the most expensive, so I would keep them as they are in terms of squad size. Both options have their place, I see the stubcarbine/power sword combo working well hunting MEQ type targets, while the flechette/taser goad will do well against horde units, and in cc they can take on tougher targets with the taser goad giving +2 to S and the taser bonus when hitting on 6s causing 2 extra hits.
For me, I actually think these guys are pretty usable with the army rules, remember that the detachment gets scout as well, so they infiltrate turn 1 + scout and with dunestrider they move very fast. They aren't assaulting turn one but don't need too, they present a larger threat to your opponent than they actually are and get him to act accordingly. With a decent save and FnP, your opponent may draw some of his heavier fire away from your main force to deal with them, which saves some of your more important units. at 185, they really are not a huge investment for the threat they give.
I see two units of them, one with swords/stubs and one flechette/taser. Each put pressure turn one and hopefully survive turn to turn two for some mean assaults. That first turn is important to get as close as possible to make use of neurostatic aura to reduce BS fire on them. These are a good spoiler unit that will do some nice damage if they survive, and if not they will draw fire away from your main force. With their aura, if they get close they will annoy your opponent enough from the effects they are having to really draw some fire. I know it sounds like they are a suicide unit, and really they kind of are, and thats not a bad thing. They will draw more fire turn one than their pts are worth, if you play terrain right too they can soak up some serious damage, and if S6 comes out, that ok, its not hitting your vehicles.
They are the first threat, and if you are going first, play aggressive with the infiltrators, get them in messing about and screwing with your opponents line to get the reaction you want. Remember they don't have grenades, so be careful when charging into cover.
The Ruststalkers are the next unit, they are going to be a bit harder to use. Stat line wise they are exactly the same as infiltrators. They also have sicarian armor, but then it differs. They have transonic razors and chord claws. These are S user Ap 5 Melee, where they always wound on a 6, which is AP2 on first round of combat, and after first round they are always AP2. The Chordclaw also makes a separate attack with fleshbane with the same above rules. Importantly they have mindscrambler grenades, which for shooting are 8" SX AP4 Assault 1, Blast Haywire Neural trauma. This causes them to always wound on a 4+. They are also assault grenades, and for vehicles are S3 AP4 haywire in assault.
Now they can replace all their weapons, including grenades, for transonic blades for free, with blades having +1 S. But they lose grenades, which is kind of a big deal. For special rules they have FnP, Bulky, Doctrina, Dunestrider, and Furious Charge. Honestly I have a hard time seeing them with a pair of blades, the loss of grenades is a steep price to pay for the +1 to S, but till tested can't say its horrible choice either.
These guys are cheaper at 160 for five and 30pts for each up to ten total, so this squad can be upped. I would recommend it too, though a full ten man is not needed either. So how to use them. Well again don't forget scout, which helps a bit, plus dunestrider, and since they only have grenades, they are moving and running, which gives them a total of 24" of movement potential turn one, note potential. Crusader does help too with rerolling that run move. So you can get these guys up the field fast if needed.
What I like about them and why I am inclined to take them, is that they basically have a form of rending, so no matter how high your T, they are still a threat. And if they can survive multiple rounds of combat, they get even deadlier. They are not the CC gods of 40k, so don't send them against those gods and expect miracles. They can compete with mid-high cc units and do ok, they'll kill a bit, but will die in the process too. Really, avoid S6 or higher if you can, as it will keep them alive much longer. But they will do well against Pfist termies, as they can lay on the attacks before they strike, reasonably most of them before they strike.
Also lets not forget the doctrina's, while the CC ones are not the go too, when these guys hit combat against a tough unit, don't be afraid to go all out and boost that WS, that will go a long way to boosting their lifespan in CC.
How I see them working well though is as a counter attack unit, as a lurker. Keep them hidden, lurking around behind your main lines, then when a tougher unit breaches it they leap forward to pounce on them. Or as your line slowly advances, they can then leap across late game to hit an objective that needs cleaning. You will have to play smart with them, but they are worth considering as they have AP2 (always on later combat rounds) and can wound anything with a T.
The infiltrators are def easier to use, but if you can get the rustalkers in to combat, they will do far more damage. They both have large threat ranges once you take into account the detachment rules as well as their own, so don't overlook them quite yet.
GG