there's no free lunch obviously...
Generally there is a one time fee of 1-3% of the transfer amount to take advantage of the offer, and also dollar limits on what you can put directly into your checking account and how often (usually like a $5k limit, once every 90 days or so).
I imagine if you just deposit in cash, you'll actually lose money as it's pretty rare to find a cash rate that's higher than the transfer fee. I've been doing this for more than 10 years now and never had a cash account that paid more than the fee, or if it did it was only 25 basis points so wasn't really worthe the hassle.
With all that said...
WHY YES! I do get these as well and frequently take advantage of them!
I prefer to use them for debt reduction, not for investment. As the PP indicated, this doesn't work if an investment goes down in value.
I estimate how much I could theoretically pay extra on say my mortgage balance over the term of the offer (12 to 18 months worth, whatever the offer timeline is, with a good margin of safety just to be conservative), and will get that much put directly into my checking account.
I just did this in late February, took $11k out with a 2% fee upfront for 0% for 15 months, and put it all toward our 2nd mortgage which is at 5.75%.
The rest are all approximate figures: I itemize deductions so I used 25% as my marginal tax rate on the interest savings of @ $1,100 which gives me a true cash equivalent of $825. That minus the $220 fee gives me savings of $605 (just over $40 a month, I use 14 months in my calc since it takes a week or two to get the money moved between the various locations)!
PLUS - it has the added benefit of dropping the balloon by @ $17k (so a factor of 1.545 times). That's SUPER SWEET!
Just make sure you're sure you can afford to pay early this way...I've been a little too aggressive a couple times in the past, and had to dip into my safety margin to hold things together for a few weeks/months.