The Opteron, which is based on the code-named 'Hammer' architecture, will be targeted at the Server & Workstation crowd offering native 32/64bit application support, an integrated memory controller, HyperTransport technology, 1-way to 8-way scalability, among other things.
The whole bunch of reviews are still a couple of hours away, though TomHW's article has gone up a bit earlier offering detailed impressions on features and performance of the Opteron. AMD itself has also posted extended information on the processor's server performance here.
Update: New reviews at Ace's Hardware, Linux Magazine & AMDZone. Also guys at Firingsquad have posted an interview with Tim Sweeney regarding Epicís 64-bit plans and their thoughts on AMDís 64-bit approach.