Mercenaries I used and my opinions:
Ogre Bulls - took once unit of 8 - it actualy worked great as it was US 24 with fear - guaranting auto-break if (or with such amount of ogres it is reather when) you win combat. Disadvantage - tend to drow huge amount of missile fire (but on the other hands it keeps your other troops live). Best use - in mostly infantry army vs Undead of all sorts. Unit of 8 with musician and 2nd weapon is 322pts, so it is still reasonably cheap
Ogre Ironguts - Used units of 3 and 4, with and w/o banner, sometimes even with a warbanner. 3 w/musician are 154 pts and are great flankers. 4 with Standard and musician are 222 (247 if you give them War Banner) and are good hammer unit. As a nice benefi those are Ld 8, so can operate far away from general. Best use - 3 if you want to use them for flanking, 4 with standard and potentialy warbanner if you want to make a hammer. Tried and worked well in cavalry army, I use them mostly to support main hammer knight unit or to attack same unit as a STank (3 ogres + Stank are US19 - that will almost always make auto-break posible).
Ogre Maneaters - Kitchen sink. But a good one. Definetly only good for a larger battles, but they can work great, expecialy if you bring BSB (they are stubborn on a Ld 8 AND imune to psychology, so they are good vs undead also)... 3 of those are 270pts, no command posible, but pack 12 x S7 attacks... Basicly they are Greatswords on steroides. Disadvantage is that they use TWO rare slots, but try them in case you did not fill rare slots.
Duelists - 9 with a pistols are 81pts and are great for arty guards. They also do job of huntsmen well. Unfortunetly musician costs 10pts so it is not a realy good investment. They are actualy good for all cavalry army as they can be left to guard arty.
Pikeman - None in his right mind is going to attack them from a front so it is good. Disadvantage is that they die like a flies from a shooting, so take some archers to screen them... unit of 24 with FC and HA (smallest number that will work) is ~300pts so not cheep either. Watch for a flankes as even light cavalry can break them with a flank charge (they only have outnumber and banner). I don't find them better then a swordsmen with a detachment and a Grifon banner but if you like them take them.
Heavy cavalry - What is the point when you have core knights, expect maybe if you don't give them barding when they are both cheep (19pts IIRC) and fast.
Light cavalry - nice and cheep, but don't overload with options - spears are all you need. Musician is 15pts, so I prefer to leave him home. Light cavalry is always nice, but recently I use 5 knights with musician for a lot of LC duties.
Griffon Legion - nice as it can give you 3rd S6 cavalry. Not extremely expensive at 26pts, and if given War Banner can be hammer unit. They die easily from missiles, so screen them with something until you want to charge. Glorious Charge is nice rule - but be carefull as you can fail charge if oponent fails panic check - so try to charge from as close as posible.
Only RoR I have used are Long Drong Slayer Pirates - they are EVIL - 20 of them are ultimate tar-pit, and can actualy win combats. And as aditional benefit they are not much more expensive then flagelants.