This speed is no panacea - it comes at the price of expensive ROMs and RAMs. Still, even at 6 to 10 Mhz the Z80 has respectable performance. It would be a terrible Windows machine, but is more than adequate for an awful lot of embedded systems. The Z80 itself is attractive due to its low cost and wide availability. It's pretty easy to find Z80s for less than a buck.
Tools are everywhere.
Though its index instructions are a little crude, it is a far better C machine than, say, the 8051. The Z80 in isolation would be a dead-end line. What makes it interesting is the high integration derivatives CPUs spawned by the architecture. All four of the processor families I mentioned earlier exist in multiple proliferation versions. Zilog's version is the Z180, which is essentially the same part.