I've gone through all the units, now its time to go through the detachment and formation rules. They do have some interesting rules and the detachment really helps this army out and makes it able to compete decently well without transports. Some of the formations are very situational, but I like most of them and only one is kind of bad, more from the pts cost. Alright time to get into it.
Since they have no Hq and can't use the CAD, they have their own detachment, and its damn good. The Skitarii Maniple, two troop choices are the min requirement, thats it. Optional are 6 more troops (8 total), 4 Elites, 2 Fast Attack, 4 Heavy Support, 1 Fortification. 2 FA seem limiting, but really they only have one FA and each squadron can be 6 strong so unless you really want a lot of dragoons, you'll be fine, and if you do want a lot more, there is a formation and you can just take a second detachment.
Normal restrictions (all have to be skitarii). Command benefits are where this starts to get great, first is Crux Mechanicus. If this is the primary detachment, you get to reroll the skitarii warlord traits, additionally your warlord gets preferred enemy. This is awesome, as your warlord transfers this to his squad, so a couple of plasma calivers will be good in the warlords squad for this reason. The other rule is Tireless advance, which grants crusader and scout (they cannot outflank with this scout though, if they have other rules that allow them to outflank, they still can).
This goes a long way to help this army out, with no transports, mobility is an issue. Here you get some help with scout. Crusader is also great for rerolling your run moves and helping with sweeping advances, though really its helpful for running mostly. Ruststalkers and infiltrators need this the most as it gets them up the field faster where they need to be.
Overall, awesome detachment, its go too. Its perfect for ally use with min requirements, and is a great detachment on its own. Its tempting for the skitarii to be warlord, as they get some decent traits and preferred enemy is quite good.
Now on to formations. Here you have the Battle Maniple, which is one of every unit in the codex. You have the same crux mechanicus rule as above as well as tireless advance. The other rule is Datalock, which grants leadership 10 to units within 12" of the dunecrawler/s. Really this isn't a bad formation at all, the ld boost is a good thing. So you do have to take ruststalkers and infiltrators, but really it's not a bad thing. You have a unit of dragoons, which only start at one, same as dunecrawlers and ironstriders. If you are looking for a reliable Ld army, and take another detachment, this is a good choice as you can keep this pretty cheap and you get good troops and the cc units are not bad. Really the Ld boost is the big thing, as if you take three dunecrawlers they cover a lot of ground.
The next formation is the War Cohort, and its three of the Battle Maniples. This is a cool fluffy formation, but its too expensive, its over 1900 pts with no upgrades to the squads, so it would be great in larger games, but anything under 2000pts and you can't play it. It has the same rules as the Battle maniple with the additional rule of the Enhanced Imperative Resolution, which allows you to use a single doctrina imperative twice, which is nice. This is a good formation don't get me wrong, its just too expensive to be used most times. So if you are playing high pts, really consider this, otherwise sorry.
The next Formation is the Sicarian Killclade, it is 3 squads of ruststalkers and 1 squad of sicarian infiltrators. The special rules are malignant suserrations which has all enemy models with 12" of the infiltrators affected by the neurostatic aura. Its a nice boost, doubling its range and is not bad. The other rule is Slaughter sprint which is a once per game use allowing you to run and assault as long as it is not turn 1. This is a quite nice rule. I like this formation, but I wish it was only 2 ruststalkers instead of three. If it was, I would consider this as a better choice, but its an expensive formation, and they don't have scout. This could work well though so don't count it out till you try it.
The last formation is the Iron strider cavaliers. Its two squads of dragoons and one of ironstriders. They have acute senses and outflank, they must start in reserves and when they come in they come in on a single roll for the whole formation, plus they can come in on turn 1. Ident confirmed, at start of first game turn nominate on enemy character, the formation rerolls failed to wound rolls against that character and his unit. Mindlinked hunters, models in this formation can always trace line of sight through each other and never confer a cover save for shooting through intervening unit. This is a good formation.
What I really like about it is that it is cheap, but can be buffed to a really large size if you want to. I will prob take this once I get to the point of having enough to take them, which will be a while. The ironstriders have precision shots so they can dump wounds on things you want to die, namely special and heavy weapons, and they won't be blocking each other. This could be a good time for the radium jezzail on the dragoons, at least one squad of them, as that reroll to wound makes that sniper weapon much more deadly.
This is a solid formation to take with another army too, it gives you some fast moving units that can do CC well and anti air in a pinch. So if you are a little weak on AA, but don't want to fully invest in a full skitarii army, this is the way to go. This unit would just work great with scions. The ironstriders with TL cognis autocannons will do great against multiple threats and target air in a pinch. Again, this is a top choice for me.
Hope this helps and gives you a better idea of the skitarii army.
GG