The savings from renting DVC points varies -- largely depending on what size villa you rent, but also based on the resort. Studios and 2-bedrooms tend to offer a much bigger savings than a 1-bedroom, for example. (FWIW, we just got back from a split stay in studios at Beach Club and Polynesian, staying on rented points through David's Vacation Rentals, and even though we paid an extra dollar or two per point to reserve at 11 months, we saved 38% at the Polynesian and 50% at Beach Club, compared with rack rate for the same rooms.)
The primary drawback is that your reservation must be paid in full immediately after it's secured, and is non-refundable, so if you cancel, you'll lose everything you paid. We sometimes get vacation insurance (through TravelGuard) to cover the non-refundable costs, if we're traveling with a large party or during a time of year when it's possible for flights to be canceled.
The lack of daily Mousekeeping is also technically a drawback -- but it's almost a non-issue as far as my family is concerned, since we pick up after ourselves.