FWIW, I found SEP2 to be slower than its predecessor, running Vista 64-bit SP2. I found the additional features very worthwhile however and by slow, I mean 10-15 seconds more processing time (in CS5). A few weeks back, I purchased a new Dell Alienware laptop with 64-bit Windows 7 Ultimate that really makes Photoshop scream, primarily due to 16 GB of ram, from my previous 4 GB.
However, ALL my Nik plugins (CEP3, SEP2, Viveza 2.0 and HDR Efex Pro) crashed Photoshop upon loading. The Nik tech team was very responsive, and all the plugins needed was a file download and installation. SEP2 was the lone problem and they had just determined that it was an issue with some of the new graphics processors (don't ask me what, I'm not that much of a techie!). They did go in remotely and after a few tweaks with my NVIDIA settings, have it up and running and it's now running great and FAST.
I am a photographer, not a programmer, so I cannot fault them or say this should have been anticipated before release but as an end user, I found their tech support to be excellent and SEP2 (plus the rest of my Nik plugins) to be working very well in 32 and 64-bit CS5.
I can't speak to the issues Mac users are having but it appears PC users woes may be over.
Best of luck to everyone out there with this...
Phil Weber