We stayed at the Doubletree for two years in a row (April 2005 and April 2006), 2 adults and a toddler. We really appreciated the extra room!! Our daughter was at an age that she couldn't sleep in the same room as us so it worked out well. If you need the extra room and you're looking to save some money I think the Doubletree is great, especially if you get a good deal like they're offering on Mousesavers right now.
The two years we stayed there we didn't have a rental car but we walked a lot to Downtown Disney (maybe about a 10-15 minute walk) and to Gooding's supermarket and restaurants in that plaza. (about 5 minute walk, but need to cross busy street). We're from the city and used to walking to restaurants and shops, so it was really nice to be able to walk places for dinner or shopping. Plus, the walkway was safe, quiet, clean and perfectly landscaped! (One of the many, many reasons why I love WDW!)
The transportation to the parts was adequate, but not great. We waited a long time for buses and the drop-off/pick-up areas at the parks were kind of a long walk. It was fine for our needs at the time, but we didn't try to make rope-drops, stay at the parks until closing or go back to the hotel for breaks during the day.
That said....last spring we stayed on property and it spoiled us for anything else!!

We'll probably always stay on-property from now on.
Have a great trip!!
PS> I just read this through and I assumed that you were asking about the Doubletree that is on Hotel Plaza Boulevard by DTD?? Hope that's right...