The Ki-61 can hold it's own against many planes in the MW arena. Not enough pilots/action there to drawn me to MW though.
In LW, I've found the Ki-61 a nice diversion at times, although it is outclassed by most planes you'll commonly find there. I've found it has two functions it can be pretty good at:
Ki-61 probably the best Japanese dive bomber in the game, and maybe one of the best JABOS aside from some very heavy ord carrying planes (jugs, ponies, 38's, etc.). 250 kg GP bombs (551lbs --- 1,102 lbs altogether). 2x20 mm Ho-5 120 rpg. I've finished off the odd damaged CV or two with Ki-61's.
The slow firing 20mm cannons I've used against GV's with some success (as well as dropping 2 - 551 lbs bombs on them), especially when Flak guns are around the area. The speed of the Ki-61 has helped in some situations, where slower ground attack planes would get chewed up.
Not sure if the cannon's for Ki-61 have changed in last version or two, last couple anti-GV runs have been less successful than my average.
Can't compare it to a Spit, since I don't fly Spits that often to rate or compare them. The Klown Kar is a fun ride at times though.