I have done this for years and have been quite successful at it. Two reason I even need to do this - desire to pay for engagement ring and pay for 150 person wedding without using my nest-egg/savings.
You need to keep a document that details when the free APRs run out, and you need to setup automatic payments of the minimum balance from your checking account so you ensure you do not miss a payment.
You need to understand when you are going to need to make big purchases of homes/apartments/cars so that you can ensure you aren't doing this right before your credit score is being checked (Need to leave 12 months without opening new accounts before doing this).
I have been able to finance about $60,000 worth of unsecured debt paying between 3-5% interest per year. We have an unsecured credit line with Citibank that charges 14% and is capped at $10,000, and Discover Personal Loans charge around 14% as well (in pre-paid interest!).
I would say, all things considering, that this is a great deal if you don't have a secured line of credit with a low interest rate.
It is something, however, that if you aren't diligent can get away from you. From my experience I would rather pay a small amount of interest while allowing my savings and retirement to grow interest (hopefully greater than 3-5%) than pay the huge expense and start from zero. But if you miss a single payment on a 0 APR balance transfer they charge you all back interest concessions plus charge you a penalty rate.
Like many of the other posts, you need discipline to make this work.