What you are giving up:
Your arrangement is with an individual, not with Disney.
You won't qualify for promotions (apparently Free Dining is the most distressing)
You will not have daily housekeeping
Your resort will be a
DVC resort - the studio rooms are a little different - generally featuring a queen bed and a full pullout. If you need two queen beds, your DVC options are limited to OKW. If you want a King bed, you'll be reserving a one bedroom - which doesn't save nearly as much money. On the other hand, for a couple that can share a queen bed, the pullout is wonderful....its like a small studio apartment.
Your cancellation policies will be different, and vary with the owner and the arrangements you make. Many DVC owners make the transactions final. If you can't go, you might be out your money.
Likewise, payment terms vary by owner. Many will ask for half up front and half 90 days out. Most can't take credit cards.
Availability can be tricky as implied. The closer you can make your reservations to eleven months prior to travel, the more choices you'll have. If you want October for this year right now, you may find not a lot of availability - particularly those Epcot resorts which are almost certainly booked up by now.
You gain a really nice room at a very affordable price.