Since the 8th edition rule book came out, it made several units more viable then when we were playing in the 7th edition. I will cover these and more.
Gutter Runners (7/10)
For me they are a must, because in almost any good list you will have either or both the Plague furnace and Screaming Bell. Which means the opponent will usually have at least 2-3 war machines/catapults to take them out. Along side the storm banner, the gutter runners will bide you more time, and will usually get your towers into battle.
Gutter runners are very fast with a movement of 6, and have pretty good initiative at 5, and due to skirmishing, have a 360 degree line of arc. They are excellent at hunting down war machines and other specific units. I will avoid taking their champion because if you are attacking another unit that has a character, they can issue a challenge, which will limit how many attacks you can get in a a specific model.
The warp grinder is an ok option, but may be too expensive to be a solid option. I recommend using either the scout rule or the sneaky infiltrator.
A descent setup is slings with poisoned attacks. I also like taking at least 2 units of 7 to better you chances of getting one out the second turn.
I give them the 7 because we are playing now with the 8 edition rules that allows the rat ogres to wreck havoc. They were almost a waste to take in the 7th edition rules, but now that you can attack with extra ranks, and perhaps if you are bold enough to go with 18 rat ogres (6x3) that could potentially get you 60 Strength 5 attacks. What!!! That is awesome. Further more, if you are spending that type of points in this unit, you may also want to spend points a packmaster skweel gnawtooth, who can potentially give this unit regeneration, poison attacks or 1 extra attack. Bringing the 18 rat ogres is also a big point sink that your opponent will eventually have to go after. And I have done it before, and will probably do it again. After saying that, what I normally will take when taking rat ogres is between 6 or 8 rat ogres along with Master moulder and a great weapon, running them up against the flanks of your enemies. With the awesome movement of 6, they can really do some damage as a hammer.
The only real cons is the frenzy of the rat ogres. Allowing opponents to bait them in certain directions, and having your pack masters killed causing stupidity among the ranks.
Plague Monks (7/10)
There are two ways I have seen plague monks used. The first being the protector of the furnace. When building this unit, I will normally take between 27 and 35 monks. Full command and a magic banner.
The second use I have seen is taking 10-12 monks and using them as a hammer option pairing these guys up with Censer bearers is not a bad option either. The main thing I do not like about the plague monks is the low initiative they have compared to the rest of the Skaven army. Sitting at 3 isn't so hot, and may prove to be vital if taking smaller units.
Plague Censer Bearers(8/10)
The Censer bearers have always been one of my favorites. As a popular choice in many lists, they are worth every point that are spent on them. Along with having the fumes of the censer, they have hatred and they hit hard. They are a nice option to take as a hammer unit, along side the furnace.
Normally I take 7-8 per unit, with a champion.
Going from the 7th edition Skaven rulebook to the 8th, they have lost a lot of their mojo, and now are a very expensive long range option to take. Mainly because they lost skirmishing ability and shoot in rank and file. They still do descent damage to high toughness opponents, but not enough to warrant how much it cost to take them. But I do give these guys more credit than their brothers Poison wind Globadiers mainly because of the range, and they can possibly wound on 2's rather than on 4+. On larger creatures though its more even, but if you need long range, these guys are ok.
Poison Wind Globadiers.(4/10)
With the ability to take a poison wind mortar, one unit of these may be worth the cost of taking a minimal sized unit. Globadiers as mentioned earlier, are not as good when attacking weaker opponents, but do still have some uses for units that have toughness 10, as they still only need a 4 to wound, and remove any armor saves unit would have. (So if those man things have their steam tanks, you now what to do)
The biggest downside is their range of 8 inches. If its possible to tie up a unit with slaves, then come form behind and get some pot shots in, i can see these guys as a very useful unit.