Listen, I know it sucks. My favorite drummer leaving one of my favorite bands (the others being Iron Maiden and Sonata Arctica) is horrible. But people need to learn to read between the lines a bit to get the real story.
Being mad at Dream Theater - Come on. Portnoy basically asked four other guys to put their careers on hold while he goes out and makes a shitload of money with a band that has a number one album. They did what anyone would do it that position. Sorry, mate, but we need to make money too. Congrats on your new gig, see you later.
Portnoy on being burned out - Bullshit. He's bitching about Dream Theater's schedule, but meanwhile adds on tour after tour in between Dream Theater tours. He wanted Dream Theater to go on hiatus so that he could make money with Avenged Sevenfold, plain and simple. His story on the press release is just that: A story. It's designed to make him appear in a good light.
On Dream Theater not being Dream Theater without him: Bullshit. If Steve Harris left Iron Maiden, that would end Maiden, because Steve writes most of their stuff and has a large hand in even the stuff he doesn't write. Historically, Petrucci has written the lion's share of the lyrics, and the music is usually split evenly among Petrucci, Myung, Portnoy, and the current keyboardist. (Rudess now, but Moore and Sherinian wrote on their albums as well.) And let's face it, there are so many drummers out there who idolized Portnoy and grew up trying to play like him that replacing him won't really affect their sound to much. I can list of dozens of prog-metal bands with amazing drummers that could fill in for Portnoy with no problem.
So, I can see why Portnoy would want to leave/take a break, but I can't sympathize with him one bit. I think Dream Theater did the right thing.