I chose a fighter with the "Wizard Slayer" kit as my main in single player. The kit grants a bonus to interupting spells when my fighter hits a caster, and a magic resistance bonus per level.

The penalty for this bonus is the inability to use any magic items besides weapons and armor. On the surface this may seem to be a fair exchange, but as I am playing the game, this penalty is proving to be harsh indeed. I can't use ANY potions, and it seems that the breastplate area is the only area that is considered "armor". So far, I have found a pair of magic bracers (bracers of defense) that my main cant use. Initially, I thought that I would only be sacrificing magic jewelry, and even that alone would have been a bit harsh. NO POTIONS? NO MAGIC ITEMS EXCEPT BP and WEAP? DANG!

Furthermore, it seems that whenever a caster is hit, their spell is interupted anyway. I don't really see where a "bonus to spell interuption when successfully hitting a mage" is much of a bonus at all. It seems to me that in exchange for giving up magic items and potions, besides BP's and Weaps, all that I am really gaining is some MR.

So, my questions is: Does this kit suck as much as it appears to, or am I missing something?

You have missed nothing, that kit truely sucks.

Yep. Throw it on the pile with the Blade, Jester, Avenger, Inquisitor and Diviner.