What's so great about PPC? There's gotta be something
"What's so great about the PowerPC?"

There are so many ways to answer this question.  It wouldn't surprise me if there is a book on the topic.  I will answer it in the most concise and simple way possible and leave the details for the reader...

The PowerPC was a great processor because it was the right chip, for the right price, at the right time.  For much of its life it was the fastest non-Intel CPU available (that was also commonly available).  Compared to todays CPU's the PowerPC is just OK but at the time it was arguably the best.  But in the current CPU market, ARM is the dominant non-Intel based CPU and the PowerPC is fading fast.  Except for some very niche applications, there is no good reason to design a new product with a PowerPC CPU anymore.

As for DSP, there are so many other ways to get fast number crunching than to use a PowerPC.  Only a few high-end versions of the PowerPC ever did well with DSP, that would be those with the Altivec instructions.  But that is old technology and there are Intel and ARM processors that do just as good if not much better.  There are also dedicated DSP chips that would be a modern alternative to the PowerPC for this application.

If you can use off the shelf PCB's then a modern Intel CPU can give you massive amounts of DSP power for relatively little cost.  If you can't use an off the shelf PCB, and must make your own, then consider ARM chips or dedicated DSP chips (or ARM's with dedicated DSPs in them from T.I.).  I say this without knowing your application, so take this advice with a huge grain of salt.