I wish I could get cashback on groceries, but that card would make no sense to me. We buy 90% of our groceries at Sam's Club and Aldi. Aldi doesn't accept credit cards and credit card companies don't consider Sam's Club a grocery store.
My wife and I started churning in October. We go one at a time and are currently on our 3rd card (halfway there). So far we've racked up 120,000 Chase UR points and are working on the 35,000 Starwood SPG points.
For those who say how do you meet the high limits:
These cards have some high limits (Chase Sapphire is $4,000 in 3 months), we had some pretty major expenses over the last few months such as a new garage door, study materials and exam fees for my wife's licensing exam (over $600 fee for that fucking exam!!!), other remodeling expenses. I also used the card for a rather large medical bill and instead of using the HSA debit card, I just filled out a reimbursement form for them to send an automatic deposit. I also have the family AT&T plan under my name which totals around $300/month, and am able to pay the utility bills with the credit cards.
We probably won't be able to churn them this quickly from now on, but I figure we should be able to do 3-5 per year (depending on the limit) going forward. That's should be a couple thousand dollars worth of passive income per year. Of course I would prefer to keep them as airline miles, but many have the option to be turned into cash/statement credit.