I've got something around 450 hours in piston-powered PA-31s, and then another 75 or so in Cheyenne IIs (which are basically turbine powered pressurized Navajos - PA-31T-620). The PA-31 is one of my favorite airplanes.
As mentioned above, around 36 GPH combined for the short body Navajos, and around 40 for the Chieftain. But it does depend on how you choose to fly it, like any other plane. We got cruise fuel burn at 50 GPH in the Chieftain when the pax "didn't buy this plane to go slow", and I got the short body down to around 32 combined when I was trying to get better economy (because I was paying for the fuel).
I far prefer short body Navajos to Chieftains. They fly better, more responsive, quieter (that extra 2 feet of cabin space puts the pilots closer to the propellers), really just everything is better. It's what the airframe was designed around, and then like any stretched aircraft, it might've been certifiable, but it just wasn't the design was originally for. They're just nice flying airplanes, and with 40 degree flaps you can do some wonderful brick approaches.
The wing lockers are nice for storage, but they do something that seems to hurt wing airflow and lift overall. What would be really nice would be a short body with the 350 HP engine upgrade.
The Cheyennes remain one of my favorite personal twin turboprops. King Airs are boring. MU-2s have small cockpits and aren't for everyone as far as flying characteristics go. Turbine Commanders are... odd. The Cheyenne is enjoyable to fly (I think) with all the positive characteristics of the Navajo, spacious cockpit, comfortable cabin, and good range. I've recommended them to more than one person, and everyone I know who's bought one has loved it.
So when it comes to PA-31s, to quote Bueller: