The leaked beta is an ISO file that weighs in 2.44GB, it features Windows 7 Ultimate Edition beta 1 (build 6.1.7000.0.081212-1400) and it's been confirmed that it's the real thing.
Adrian Kingsley-Hughes from ZD Net has downloaded the beta and states it's a good one. He reports the beta is of excellent quality and exceeds the quality of any other Microsoft OS beta he has ever handled. He hasn't benchmarked the beta yet, but says the performance is exceptional, the operating system feels very snappy and responsive. Adrian also wrote a short review of the beta, you can check out that article over here.