I'd like to add to this what I've learned. I've only fought against a small, consistent force of Skitarii, and I've not enjoyed my experience so far.
My Skitarii friend and I play at 500pts at the moment, on a board 4x4ft square, and so we don't have 'long' and 'short' edges, but as we grow in the hobby, buy more figures, build a games table etc. we'll play at higher points level and experiment more.
The force I use consists of 20 Fire Warriors, a Commander and his team of 3 Crisis, all donning mid range weapons (Plasma and Ion). He brings 20 Rangers and 5 Ruststalkers.
Let's start with the Rangers, as they anger me less than the Ruststalkers
Rangers. They come stock standard with Feel No Pain 6, Pulse Rifles with 1 less Strength and AP, Precision shots, Move Through Cover, get to Scout and get access to some pretty good heavy weapons. My friend brings the 60" sniper, one per squad. Now, for 2 points more per model than Fire Warriors, I feel like they get a little more than they should, but I can't change the rules, so I had to adapt.
They're painful to deal with because if you want to kill them with fire warriors, you'll need to set it up pretty well. If you get into Rapid Fire range, then they're also in Rapid Fire range. Our Pulse Rifles wound on a 2+, but without markerlights we only have a 50% chance to hit, so we almost need to rely on Rapid Fire to kill them or scare them off. If you fail, they're going to retaliate with BS5+Precise AP4 shots, so almost all of them will hit, and if you're not in cover, then say goodbye to your beloved Shas'la.
IF you manage to cause a pleasant number of wounds, they have 4+ armour saves, then a 6+ Feel No Pain to make you feel bad about having a recent codex renewal. On top of all these great things they come with, they can get a -1 to cover upgrade for their leader, and a 5+ invulnerable save for him as well, just to make your life harder. Say goodbye to your Ld8!
When they've finished off all your fire warriors, then they turn on whatever else you have left. In my case, it's my Commander(because precise.... Yeah, his dice rolls are really good).
The Ruststalkers..... Oh the Ruststalkers.... They are the bane of everything I enjoy about this game. They get to move an extra 6" per turn on top of their move/run or move/charge. They have Feel no Pain, all have invulnderable saves, albiet 6+, have 2 wounds each and have "rend" style weapons that are insta-rend if you manage to survive one round of combat. On the plus side, their armour save is still only 4+, so in the case of my Crisis Command team, its not hard to force them to take their miserable invulnerable saves with no Feel No Pain, but if I fail to kill them or make them run off the board, so help me Aun'va...... They're almost guaranteed to be in melee combat with something on the second turn, and if they aren't, they're either in your face, or dead. They make easy work of anything I've thrown at them, so if I know I'm going to get the first turn, I'll generally try to Deep Strike and blast them with high strength Plasma and Ion. The only downside to this is that it's a huge gamble, and even if I do kill them, I have the 20 odd rangers to deal with....
Those Doctrina Imperatives are rather quite annoying, the Rangers out Fire Warrior the Fire Warriors, and the Ruststalkers are infuriating speedy balls of flailing metal limbs and swords. The Skitarii peak for power really early on in points, which is sorta the opposite to Tau, because all our best units come out at high point games.... Frustrating.
My personal experience with the Skitarii hasn't been a good one, but I really do hope that at higher points values I can level the game a little more, what with bringing Broadsides, Riptides and more Markerlights than even DARPA can imagine