+1 for what David said re Harry's videos - they're invaluable and give you an idea of the power of this beast. It certainly takes a bit of getting used to after PS and I still find myself doing some things in PS although I should force myself to stay in PI probably.
IMHO PI is worth its price tag just for:
- DBE (correcting for gradients, poor flats, etc)
- great noise reduction routine with built in masking
- very powerful deconvolution tools
- curves for colour saturation
- blinking script (done by a User)
- Cumbersome but powerful stacking
The programmer continues to power ahead and the online community is very supportive and helpful. I believe the latest version now has some decent documentation, but I haven't had a chance to explore it yet this year...
In the end it is another collection of tools. You need to invest the hours to learn how to get the best from it (I'm only in the foothills of Everest at the moment...

)