Personally, I find the a3 mercs to be too useless to be worth the trouble. Then again, I'm not on ladder and can't get spirit for them.
The a1 mercs, the archers... they can actually be quite deadly, but it requires expensive bows to make them do so, as well as an aura. My hammerdin currently has a rogue scout who does 5.2k damage with each shot, using a shael'ed wf, crown of theives, and... duriels shell. If I were to get her ed/ias armor and helm... well, that's out of my price range. But that.. would be disgusting.
She kills some other players in pvp. It's funny watching people try to catch up to me then complain because my merc's been peppering them with arrows the entire time. :lol:
As for a5 barbs... yes. Those guys are a lot of fun. My main reason for liking them? They can use different equipment than the a2 town guards. Getting the same polearm time after time can be tiresome. It's nice to change it up... I have a nl one using an eth swordguard. If only it were ladder, I'd up that puppy... *drools*