The same can be said about the people who bash Raimi's films, and just movies in general. They'll bash everything. I haven't seen 500 Days of Summer, so I really can't say whether he's a great choice or not.
I haven't bashed Spider-man 1 or 2 because they are well made movies. This was going to happen, Tobey MaGuire is old. Sam Raimi failed last time, and yes I don't really care about the excuses of the studio making him doing stuff. It was still his responsibility, and he failed. And I respect him for leaving, you should too.