This goes for all the M&M games. It never seems to work in 4, and I've played 6 and 7 a bunch and it never works in those either. Is it any good vs. anything?

Yeah it works fine... When you're not casting it =p

In 6-8 I think I have gotten it to work a few times. Half the spells in 6 don't do anything! Any thing that's supposed to cuase a status effect on your enemies won't work! I have no clue how the developers could of screwed that one up. They fix that problem in 7 and 8 though.

In MM 1 and 2 Finger of Death is a lvl 5 spell that is made redundant when you get access to the lvl 6 spell Disintegrate. I never had any success with Finger of Death, but Disintegrate was pretty good.

But generally only spell that boost your own party or directly damage your enemies are useful in Might&Magic. Sadly most "clever" combat spells have too low chance to work to be useful.

Yeah, most games of that area aren't balanced extremely well. Heck, a lot of games now aren't either.

I remember plowing through M&M6 with 4 druids casting sparks. Dominating. :)

In MM2 the Disrupt never seems to work for me either.

In Xeen, Finger of Death only affects [url=http://xeen.wikia.com/wiki/Category:Animal_Monsters]animal[/url] and [url=http://xeen.wikia.com/wiki/Category:Humanoid_Monsters]humanoid[/url] monsters. It automatically misses everything else. Make sure you're using it on the right type of monster before deciding it just doesn't work.
Monsters also get a saving throw based on their monster ID. The insane beggar (ID 12) has a much higher chance of getting hit than the witch (ID 62) for example.
Also keep in mind that it does 50 damage, solid. That's typically enough to kill a lot of monsters but it quickly becomes not enough for a one hit kill. The giant toad near witches tower, for example, has 90 hp. I just confirmed that it does in fact hit a giant toad, reducing it to 40hp remaining.

In Might and Magic VI, I've used it on peasants before, but it's not nearly as fun as armageddon.