If you want a cheap hotel then it is better to stay on site. Disney Value resorts will give you much better service for only a few dollars more than offsite roach motels.
If you want a 3bd house or nice 2bd condo then offsite is 50-90% cheaper than a Disney deluxe or suite accomodation.
Bingo.
In our specific case, sharing a single hotel room between the four of us simply is not vacation. I can do that for one night,
maybe two. Anything longer requires separate sleeping spaces for adults and kids---at a minimum two hotel rooms or a 1BR suite. But we strongly prefer 2BR and kitchen facilities.
We are normally late February visitors, during Peak Season for Disney resorts. The absolute cheapest way to meet our minimum requirements onsite is two Value-class resort rooms. Some years are better than others, but a typical year will see discounts of about 15%. For 2009, that works out to about $1,700 for a week's stay. If we were to rent a 2BR OKW unit from a Member, it would be about $3,000 at $10/pt for the same week.
For less than $1,000 (usually, much less) we can get a 2BR offsite that is furnished comparably to that DVC unit. Even if you account for rental car and parking, that brings me to $1,300, give or take.
We do stay onsite, but only when we can get a timeshare exchange into DVC. We've done that several times now, and the cost is about a wash compared to staying offsite. At that point, staying onsite is great, because it doesn't cost us anything more. We even took a 1BR OKW villa once, but it wasn't as nice as we recall the offsite 2-3BR units being, and I don't think we'd do that again.
At the end of the day, for us, the benefits of being onsite were tangible, but not exceptional. We mostly have our own car, and take the bus only a few times each trip. We didn't make much use of EMH. Package delivery has been hit-or-miss in terms of timeliness and convenience. Room charging makes no difference to me. Service and amenities are comparable to the better offsite resorts.
Our next trip is going to be this summer at Wyndham Bonnet Creek, so we'll be offsite for the first time in a couple of years after several DVC stays. It will be interested to revisit the comparison and see how it feels to us.