I am playing a Lvl 6 Sorc / Lvl 10 Harrower in one campaign. It's really cool. Essentially you have
- a full caster progression class
- a spirit deck (supernatural attack, unblockable)
- a mechanic that lets you boost spells (more damage, higher CL, heal yourself etc.)
- gain some spell-like abilities like divination
- get special options with all kinds of cards, e.g. get more from a deck of illusions or a deck of many cards
- various bonuses/buffs through the harrowing that last the entire day (my party has profited a lot from having +1 to AC or saves etc.)
I play it like a a gambler, with some uncertainty as to what exactly happens...
Then there is the harrowers handbook, quite cool. Especially the monk version is rather powerful.
Hope this helps :-)