I owned the original PerFourmer and the MkII adds a lot of really useful features like PWM and osc sync.
My only real complaint with the new model is the price, it's almost double here in the USA compared to the original (shot from $1100 to $1900). I don't know if UK/Euro pricing is more reasonable.
The polyphonic mode is very retro, as Sir Ruff mentioned you can't really get all four voices to sound identical and doing tweaks while playing means you have to tweak all four. On the other hand, using it as a multi-timbral analog mono is very interesting, stack up voices on one channel to create complex sounds, use it as four monos on four channels, it's pretty flexible. Because it is analog, the FM mode is mainly for noise-making, the osc drift makes precision FM programming impossible. It's certainly useful if you will be sequencing it most of the time.
I listened to Hatfield and the North at Rainbow. They were very wonderful and they made my heart a prisoner.