Others may disagree, but here's my take on Early Entry.
The only time when it is really beneficial is when visiting the Magic Kingdom with small children. At MK, Fantasyland has a number of notorious slow-loading rides like Dumbo and the Carousel. Also Peter Pan and Winnie the Pooh are very high-demand attractions, causing lines to form early. Parents who can get to the park early and get these rides out of the way before the general public is let in will make out well.
At the other three parks, the big attractions all have FastPass. Kilimanjaro Safari at AK, Rock N Rollercoaster and Tower of Terror at MGM and Mission Space and Test Tract at Epcot. These are all rides you can conveniently experience with FastPass rather than rushing to Early Entry.
If you do decide to take advantage of early entry, you are correct that you will probably find larger-than-normal crowds as the day progresses. Hopping parks can be easy or difficult depending on the starting and ending point. Using the monorail to get from MK to Epcot is pretty easy and would likely take no more than 10 minutes.
If you want to do Early Entry, make sure you map out your timeline carefully. If the park opens at 8am, you probably want to be at your resort bus stop no later than 7:15. If you're like me, that means setting the alarm for about 6:15-6:30.
Can you tell yet that I'm not a big fan of Early Entry?

Sorry, but getting up at 6:30 just isn't my idea of a "vacation".
If you decide that Early Entry isn't for you, the best course of action is to simply skip the EE park on every given day. That way you don't have to worry about getting up extremely early, changing parks mid-day, dealing with larger crowds. Here's a web site you can try that will give you recommendations for the best days to visit individual parks:
http://www.touringplans.com/BestDays.htm