If you learn to play w.out them, you won't notice their effect whatsoever, however if you come to rely on them, they will be your crutch/handicap and when they die, so do you. I prefer to make sure my toon is strong by him/herself before getting a decent merc, then once I get one, treat them with gear I'd put on my main char and watch them wreck it like no ones business. Word of caution: a1 mercs require either tanks (i.e. summons) or caster-type that does not engage upfront or they will melt instantly. A2 mercs are sturdy as hell for all classes, however not most desired. A3 can be one of most powerful without doubt. And I heard a5 merc is by far the best for most builds, but I've never given one a decent chance to say one way or the other. In the end, pick a merc which skills best benefit your own, but make sure you are powerful on your own first!