The passive is alright...but the stats are beastly! As it looks, it's a mix of Sheen and Glacial Shroud, the combine cost is only 640, the aoe slow field didn't seem particularly powerful, the aoe Sheen proc was pretty sweet...it's a good item for tanky derps or tanks who want armor, CDR, mana, and AP. Also, I'm sure it'd work out great on tankies who have area of effect abilities. I used it both on Skarner and Naut and it does what you'd think it does - make you tanky, give you mana, and make you hit harder.
When would you build it instead of Triforce? Haven't figured this one out yet, but for every beefy champ I've played so far that I would normally grab Triforce on, I would get the Fister instead.
Side note - Saw an EZ running Fist...not sure what he was trying to do, but I'd imagine his Q having an aoe affect would be lulzy...not to mention he'd be tankier.
Another observation - There are several more items that increase movespeed now, so movespeed quints, even a single one, will go very, very far.