I've been looking at the new codex, and I thought this thread might be a good place for an idea or two of my own...
First off:
The 'Mob Rule' applies to Runtherds, and nothing in the Ork codex says a grot is not an Ork. In fact, the introduction of the book classes gretchin and snotlings as 'sub-races' of Ork, implying that they count. You could well argue that Gretchin mobs with Runtherds are normally Fearless - Runtherds don't mind sacrificing more grots until the meat-shields start running low...
If you really can min-max Runtherds (and I think the rules suggest you can), a Warboss accompanied by 3 Runtherds and 10 Grots might be a very potent assault unit coming out of a Battlewagon - and you were going to have a Warboss anyways, weren't you?
However, I like the looks of using Grotsnik when using Gretchin.
Grotsnik joins a unit of 30 Grots + 3 Runtherds = Fearless Grots with Feel No Pain (not that they'll ever benefit from it, but still...) + Cyborks wherever you want.
That tarpit-of-MC-killing works extra-well this way, and who doesn't like Cyborks?
Cybork Grots.
If nothing else, these guys would be incredibly funny to see on the field - and they'd be a riot in Apocolypse games. All those instant-kill weapons flying around, and a 95-point block of Gretchin+Runtherd can survive being hit.
"Your Volcano Cannon hit my Gretchin? Well, only twelve members of the unit survived being hit. Yes, I know it auto-wounds with no Armor Save allowed..."
For the highest amusement factor, have Grotsnik champion some Cybork Gretchin and march into the Imperial Guard's massed lasguns.
"60 shots? You hit 30? 20 Wounds? Let's see, 6 Cybork saves, and 7 FNP saves... You killed 7. Is it just me, or do flashlights suck?"
Yeah, for the price increase you could have almost three times as many gretchin, but lets face it - normal gretchin disappear at the rate of "how many did you have there?" in a turn (and that would eat up another two Troops slots in normal games).
You can have Cybork Grots with a fair Invulnerable save for less than ten points.
Maybe I'm just one of the guys with a soft spot for grots, though...