The wife and I opened 21 cards over the last 2 months. We'll earn about 1MM miles (which will be cashed out as $3000 cash to our bank and about 700k miles).
The other problem is if you start getting crazy with all these cards and make a mistake 1 big late fee or interest charge will wipe out much of your gains.
I doubt it; a $25 late fee would barely put a dent in the $3000 (and, of course, not affect the miles.
I also only give there about a 5% chance of that happening, I'm tracking everything with a spreadsheet. I expect the total hours of work (counting the manufactured spending) to be around 20.
We'll see how much it affects our credit score. The inquiries will stop affecting the score after a year, so I'm not too worried about it. Average age of accounts will be lowered somewhat, but I think that's about 15% of your score?
All in all, I'm temporarily selling maybe 30 points of credit score to make about $15,000, and I expect to get those points back within a year.
It's definitely worth it for us, but YMMV.