Isn't different than the first generation X1800 XL's (just like the one I got). They are early, un-stable X1800 XT's. The Core can't go higher than 545 Mhz to 550 Mhz without crashing the system or hanging it even with proper voltages. But the first XL's have the XT's Memory (although at lower voltages, and different circuitry, so not to reach 680+ Mhz without a single issue, but physically and technologically speaking, the Memory is the same).
The 2nd generation XL's Cores were "fixed", as I've heard, and read.
Don't worry even ATi is willing to sell crap (or should I say un-finished or un-completed products) for profit. Not to mention that in R520's case, it had to be rushed to finally come out in "due" time after all the delays, exactly because of technical issues regarding stability with the Cores, anyone remembers ? I do.