Here's my take:
As a kid, we always drove in (lived in GA at the time) and stayed off-site in a cheap motel.
As an adult, we stay on site (fly in from NY) and for us, there's no comparision. Staying on-site is just so much more fun and magical.
We love the following perks:
1. Not having to rent a car, drive, park, return the rental, etc.
2. Not having to fuss with car seats.
3. Not having to drag luggage around and deal with baggage claim (at least while in Orlando), including being able to check bags at your resort.
4. The theming and amenities of our favorite on-site resorts (which include most of the moderates and deluxes).
5. Being close to the parks and feeling immersed in the magic.
6. The feeling of being on an all-inclusive vacation (although, unless you have
free dining, the dining plans are an ever increasingly bad deal).
We don't care much about EMH (in fact, we usually try to avoid parks having EMH).
As far as the resorts themselves, while not meaning to sound snobbish, we don't care much for the value resorts (theming is too garish, plus we insist on queen beds and don't care about CS food courts as we like to eat TS).
We think the moderate resorts are the best bang for the buck on proprty (particularly POR, POFQ, and CSR), as we find the theming and amenities to be very attractive for the price. In fact, coming from NY, rack rate at these during non-peak seasons doesn't seem at all overpriced to us.
In contrast, we find rack rate at the deluxes to be outrageous, even during off-season times. At these, we believe you're mostly paying a huge location premium, as the quality and attractiveness of the resorts alone does not justify the substantial price increase over the moderates.
All that being said, I'm not sure staying on-site is worth it during peak seasons, because I believe during those times, all resort classes are overpriced (plus, no discounts are generally available). However, we always try to go during the off-season, which tips the scales toward staying on-site. Especially, if we can get a good discount.
Generally, we're of the opinion that a moderate resort during an off-season with free dining is, by far, the best deal for staying on property. However, we can be tempted to stay deluxe if room discounts are available that are substantially greater than free dining (free dining never temps us to book deluxe because we won't pay rack rate for deluxe under any circumstance, even under value season rates). But we view this as a definite splurge as, in our view, even a 40% discount at a deluxe is not as good a deal (practically speaking) as free dining at a moderate.
Granted, we realize that even when going in the off-season with discounts, we're still paying a premium to stay on-site. But we believe that under such circumstances, staying on-site is totally worth it.