I've heard/read complaints that Egg Beaters are hard to get used to because the engagement is a lot different than SPD. People talk about feeling like they're going to suddenly pull out of the pedals because the unclipping is really vague. I've also heard that reliability is an issue.
They do seem to be a much better design for mud, and I've been in CX races where it was impossible to get clipped back in because the pedals are jammed. I'm intrigued by the design, but I'm also not racing at a high enough level or in mud races frequently enough for this to be a big factor so I've always just stuck with SPDs.
I'm curious to hear opinions of anyone who's made the switch.