I used to use a suicide grunt rush to kill covies in 2v2 with a forge arby combo The covie base would be gone under 230 and its hard to see it coming cause its so fast. Its easy to stop if u know its coming and we never did it on people we just played if we got matched up against them again.
The first good people we tried it against were two generals, they used brute anders, hit my base, and brute's base was gone at like 230 on docks cause it took me a while to get there. Brute kept trying to rebuild but was eliminated and then anders tried one last attack before she quit.
The only problem I have with them is they are glitchy as hell. In 3v3 I try to put them in a reactor with a prophet or brute near it and 90% of the time they attack the covie leader. There is nothing I can do but watch them be retarded. I actually had a situation in 2v2 where I needed one more grunt to kill the covie base and was hitting it with my arbiter so the human wouldn't heal it but my grunt shot the enemy prophet for like 30 seconds instead of walking into the base whiel the prophet shot my arbiter, it was stupid.
Now my 2v2 partner just uses them to kill turrets and bases that just built but sometimes they are still glitchy.
Gamertag:Ctl Alpha
napalm beats the arbiter
real men can counter the counters to their units with godlike micro