First off, we have no model yet. So pics are rather pointless. From the pic we have in the codex though, I am excited about what this model will look like. (i've seen quite a few conversions so far, and haven't found even one that I like)
A Grotesque stats out at the following WS4 BS1 S5 T5 W3 I4 A3 Ld3 Sv6+. The leader of this pack gets one more Attack and one more leadership, both at 4. They come with night vision, power from pain, altered physique, berserk rampage, and bulky and start with Gnarlskin and a close combat weapon. You can take them in groups of 3-10, and one grotesque can exchange his close combat weapon for a liquifier gun.
Taking an Aberration (unit leader), you can also take a venom blade, mindphase gauntlet, scissorhand, or flesh gauntlet. If you do take one of these special close combat weapons, your aberration will be the only grotesque to benefit from having an additional close combat weapon so don't forget your +1 attack for having it. If you take scissor hands for instance, your aberration will get 7 attacks on the charge, always wounding on a 3+ with a re-roll for any toughness's 5 or less. If you can find your second pain token, you are getting all this on I5, and one extra strength, so you can even re-roll your to wound rolls against most monstrous creatures.
Grotesques can also take a Raider as a dedicated transport. Remember they are bulky, so only 5 will fit. If you want an independent character with them, and you will, only 4 will fit.
Berserk Rampage. If your unit of grotesque does not include an independent character at the beginning of the movement phase, roll a die. If you roll a 1, the grotesque go on a murderous rampage, inflicting 2d6 S5 AP- hits on all units within 2d6 friend or foe, before being removed from play. Vehicles count as being hit on side armor.
Now getting them to berserk rampage when wanted is a bit tricky, but I can see how much damage they would do to a de-meched army. Thus far, I haven't been able to get them to do this, and haven't seen anyone else try it. However, flying them in on a fast moving raider 24" + 2d6 and letting them go into your opponents backfield, isn't such a bad idea. Or deepstriking them in on their raider. Laws of averages say you should be able to get them to do this once a game. However, beserk rampage is not a tactic to rely on, so is probably not the best tactic to put into your arsenal.
Altered physique. The most obvious power of the Grotesque is it's strength and toughness. It is beast (not the unit type). It comes with 3 wounds a toughness 5, and starts the game with Feel No Pain. The one thing to watch for is Vindicators. That could easily ruin your day. However, Grotestques are not really that expensive. 3 wounds for 35pts.
So how do we use these monstrosities?
Giving them a liquifier gun is almost a necessity. If you have small groups of them, 3, then get yourself the extra AP d6 flamer. If your Haemonculi travelling with them also has one, that gives you 2 template weapons to scorch your opponent with before you charge into the fray. This also gives you 2 pain tokens for the unit
One important possibility is Urien Rakarth. If you take him in your army, your Grotesque's can be upgraded to S6. Attach him to the Grotesques and you immediately start the game with 2 pain tokens and become S7 on the charge.
I prefer grotesques in small units of 3 with a liquifier and aberration upgrade in a Raider with a Haemonculi. Another option would be 10 Grotesques with an aberration coming out of a webway portal. Nothing quite says ouch like that. On the charge with Urien Rakarth, 10 grotesques can deliver an astounding number of attacks on the charge, that's 43 S7 attacks at I5 WS4, and then Urien's 5 attacks at S6 I6. This is of courese a 400+pt unit. A little spendy and not counting Urien.
While I like the idea of Grotesques, really their biggest issue isn't their strengths, it's what they compete with in your valuable elite slots. Kabalite Trueborn are an extremely good elite choice and wych enthusiast will enjoy Bloodbrides. That will leave grotesques as pretty rare on the table top.