I've done full tank jungle Fiddlesticks and jungle Garen.
Full tank jungle Fiddlesticks (or Cindersticks as I call it) focuses on the large amount of innate CC that Fiddlesticks has between his fear and his silence, the initiation power of his Crowstorm, and the sustain on his W. Of course the sustain on W isn't that great without any AP, but it still gets to be pretty useful once you get Spirit Visage.
His clears are get to be super mana efficient while still having a lot of sustain because of the clear speed added by Cinderhulk. You can clear most camps reasonably fast once you get Bami's Cinder/Cinderhulk, you have tons of sustain even with a rank 1/2 Drain, and you stay at full mana because of the passive mana regeneration on Hunter's Machete and its upgrades.
His weaknesses are obviously the fact that he doesn't have the tankiness that a lot of other tanks have (especially the lack of a tank steroid and magic resistance scaling), and his sustain doesn't scale super well into the later stages into the game, but other than that it's pretty solid. As a side note, I somehow managed to have a 100% win record with Cindersticks, even in games where I didn't do too hot.
Jungle Garen is more fun than anything. His first clear is pretty slow, but after that he clears reasonably well, and can indefinitely farm the jungle after level 11, when he loses the restriction of having jungle monsters break his passive.
Later on, he becomes the tankiest champion on the map with the % health on Cinderhulk, % resistances on W's passive, and % damage reduction on W's active, not to mention he deals enough damage after building Black Cleaver to still make himself a target that his opponents need to deal with.
The problems with jungle Garen are that he loses his strong early game. He can't gank well in the early game, nor does he have enough sustain to allow him to gank consistently unless he wants to camp the lane long enough for his passive to kick in, or if you want to walk across your jungle to take camps (ie. go straight from Gromp to Krugs).