They are very, very weak. They should be at least on par with Vine Ogres IMO, as the path combination is quite rare, and there isn't much synergy besides Vision's Foe.
hes great if he can be controlled or if you only expect to use him short term (and he is quite fun to use) but i agree he isnt exactly a good investment (if you really want to use him bring a guard of mind enslavers/charmers with penetration items so if he goes wild you can recapture him for a while)
In unmodded, I didn't think Will o' Wisps were bad. Not that I used them a lot, but mainly because I was just doing other things or didn't often have many mages who were suited to doing it.