I think this all depends - as with so many other things - on a few variables. When are you traveling? What type of Disney person are you (i.e., do you want to see and do absolutely everything in commando fashion, or are you someone who's been there, done that, and is looking to do a more limited selection of attractions at your own pace)?
Personally, I'm the latter. I like sleeping in. I go during the off-season, so the parks aren't all that crowded to begin with and, if luck is with me, even the most popular attractions involve little more than a 10 minute wait. So it's pretty rare that I'll actually use the Early Entry (well, maybe on one day, so I can hit the Magic Kingdom and do consistantly crowded attractions like Peter Pan, Winnie, and the mountains).
But if you're going with a "must do everything" list, and it's the crowded time of year, I'd definitely say "take advantage of the Early Entry" system. Now, keep in mind that if you're at Disney during the peak seasons and the resorts are filled to the rafters, there's every chance that the parks will be too, as everyone else at the resorts has the same idea: "Let's get there early!" Sometimes, it actually makes sense to go to a park that's NOT featuring Early Entry on that given day, figuring maybe all the resort guests will be crowding that park.
Gee, guess that's really no help at all, now is it!
Richard