I recently IO'd my Psi/Fire and he might be my favorite hero to date. Single Target is fantastic along with great AoE for PvE.
What's made the combo really fun is slotting Ragnarok: Chance for KD in Psi Tornado. I lead with it every time, sometimes even ahead of the group. Very rough estimate, but it feels like 80% of the group gets knocked down, including most bosses (Warhulks and DE bosses give trouble). If Psychic Wail is up, then I go Psy Tornado --> Psychic Wail and then just pick off the bosses.
Adding to the KD fun (plus survivability) is slotting Fire Sword C