It's been completely re-written to read and write EXRs as fast as possible, taking advantage of all the memory you've got on your system (including you 64-bit Windows users).
There's also a new "ProEXR EZ" plug-in for people who find the whole multi-layer EXR thing a little overwhelming but want the extra control ProEXR gives you over alpha channels. ProEXR EZ even lets you put the Alpha in the Channels palette, if that floats your boat.
Most users will want to have either regular ProEXR or ProEXR EZ installed, not both. But if you do install both and open a file, ProEXR will be used unless you force EZ in Photoshop's Open dialog. When saving, you can easily pick one or the other.
Please send in bug reports and comments so we can make this the most solid release yet!