Pros & Cons of staying offsite

Advantages of staying off Disney property

Since you'll probably have your own car, you're free to come and go as you please.
Some area resorts offer free transportation to and from Disney theme parks and other Orlando area attractions such as Sea World and Universal Studios.
Rooms sizes for similarly priced hotels can be 1/3 larger (or more) than Disney's rooms.
There are many rental homes and condominiums in the area that provide 1,000 square feet (or more!) for the same price as a Disney hotel room.
Rental homes and condos offer full kitchens and all of the conveniences of home.

Unless you drive to Disney, you're pretty much forced to rent a car. This can offset some of the savings of staying offsite.
After a long day at the parks you may just want to hop on a bus and be dropped off at your hotel without having to pile into the car and navigate back in the dark.
Depending on where you stay, some of the parks may be a ½ hour drive or more.
If you buy souvenirs in the parks, you'll be stuck carrying them around with you all day. (Or course, if you purchase your souvenirs on the way out of the park, you avoid this problem.)