They are crazy busy, but depending on what you want to do you can manage it. This past weekend I did both days, and was in and out within an hour both days.
If you want the celebrity autographs, as Master Yoda said you need to wait for crazy long lines. I find that silly for me, I just don't need to spend my Disney vacation waiting for VERY minor celebrities to sign a paper lol.
However, if you want the merchandise and the photos with the SW characters, if you are there at opening you can do them very quickly. I was able to arrive at 8:30 AM, get in the park, sneak around the back (everyone heads towards Star Tours, I was the only one I saw both days who knew to cut across in front of Little Mermaid) and see all the cool characters and go to Jabba's Hutt for the limited merch.
I was shocked that I was the ONLY person in Jabba's that day as they opened. There were rows of CM's just creepily smiling waiting to help me. I even got apologized to for having to walk through such a long queue to get in; I told them it really was okay, 'cause a few seconds of walking through an EMPTY line was much better than people waiting many hours to crawl through it.
I also was able to fully access both Darth Vader and the Emporor. I actually felt bad for Palpatine, as the few little kids who had strayed over were terrified of him and he only got someone every couple of minutes to take pics with him. Both Vader and the Emp were easily accessable for at least the first hour the park opened while everyone else was in the Star Tours area.
I had a great time for an hour or so (the music is great, and the stormtroopers on the street are as well) and then left. I found going in the way I did I saw everything I wanted to. To be honest, except for Chewie the rebel characters are boring in photographs - i.e. it's obviously not Carrie Fisher or Mark Hamill, it's just a guy in a costume. Darth, Palpatine, Boba, etc. are much much cooler to be photographed with IMO, and so easy to access early in the morning.
Good luck!
AEfx