I've been playing with mine for a while now, and it's gorgeous. The hardware is fantastic, but the software is very much in the early stages - the Amiga emulator in particular is a pain to get working and takes as long to get to choosing a game as it does to set up WinUAE ;-)
I've run mine for a few hours at 1GHz, without problems - I don't know how far it will go, and I've not tried over-volting yet. However, none of the CPUs are guaranteed to go above 600Mhz.
The problem with the CPU spec is that they're a very small company and updating the CPU core during production would have meant more delays as the board was redesigned and old boards scrapped. For a small company they've done a marvellous job. The way I see it is this - if the CPU is too slow, what does that make the CPU in the DS or the PSP? Both platforms are still selling.
I really, really don't regret buying this.