when you get your void, just use it until level 30 or so. dont even invest in imp spells (save your money).
HOWEVER
once you get into 30s, you will find that investing back in the imp is worth it. at 40 or so, you can even solo with the imp if you are set up for melee.
succubus is ideal for grouping, although the imp rocks too, unless the group is weak on tanks (then pull out void). likewise, higher levels when you are going against magic chars the felhunter becomes valuable. but the imp can own in groups too.
a lot depends on your playstyle and how well you manage melee.
i myself rely heavily on staff and melee, cuz i like to save mana whenever possible.
so, again, dont even invest in the imp until you are in the 30s and can get higher rank stuff. the imp rocks for grouping too.
once you hit 30s, you also need the imp available and boosted up for pvp.