Cell was designed for heavy duty processing at low cost and relatively low power.
At that sort of thing it was *incredibly* fast. On HPC benchmarks it was getting 30x gains over x86 processors. On FFTs it was beating the P4 by 100x.
On the things it's designed for it's probably still competitive with top end x86 processors.
I have a dual core i7 MacBook Pro and I wish it had a Cell - for the audio apps I use (e.g. soft synths) it'd be far faster.
The PPE is a G4(ish) class processor, it's in-order so performance is rather erratic (it's faster at some things slower in others).
The SPEs are DSP like cores. They require more involved programming but it's not exactly brain surgery (it's a variant of AltiVec code). It's easier to get something up and running on the XBox 360 but to get high performance it's no easier - you use exactly the same techniques you use on the SPEs.
Cell is rather dated now but it fitted into the Amiga model very well. If you remember the Amiga was powerful because of it's chipset. The SPEs would work in much the same way. The main CPU would run most stuff but hand the heavy lifting to the SPEs.
However after Sony's lies about keeping their platform open, there's no more low cost Cells. There's no point even looking at them now.
