Let's have a look at Night Runners. One of the most underestimated, and at the same time overestimated units in the Skaven List.
In stats, they are exactly the same as ordinary Clanrats, but without the armour. They do have some interesting weapon choices though, and ofcourse they are Skirmishers.
In the few battles I have used them, they always amaze me, by their successes as well as their failures. I have seen them stop a Undead Cavalry charge once and slay them all, and once where they were furiously hacking away at Empire Cannon Crew, only to flee off the table, leaving all three crewmembers alive.
When I field them, I usually have points to spare, and could do with a fodder unit that might just have its uses too. Frequently I use them to shield a Warplightning cannon (if I field one - I'll talk about that one soon enough!), able to stop any charges long enough for the cannon to get away, but still let the cannon shoot between two models.
Also, I like to use them just to annoy my opponent. I hurry them into some sight obscuring terrain like a forest near to my opponents battleline, to prevent them from marching, and possibly end up behind enemy lines (hit on some lone wizards of warmachines).
Have them hide away most of the game in a faraway corner, of near the centre of the table, only to speed out in turn 5 or 6 to (dis)claim table quarters.
I usually take 10 of these guys, since 20 is way too bulky for any skirmish unit to move unhindered, and less is pretty useless. I do not know if there is a preferebce for either slings or stars, I vary them greatly. If I intend to use them as shields, annoyance, or claimer, I hardly give them any weapons. But sometimes, often when I can spare the points, I give them at least the extra handweapon, and slings for the better range than stars.
Rumour has it that these units, and all other Clan Eshin units, will be carrying poisons and the sort in the new edition somewhere end of this year or first half of 2009. Let's hope so

It would make them a lot better and more usefull, as at the moment Clan Eshin is the least played clan at tournaments.