WHM/SMN - Useless before level 50. Auto Refesh cannot be used then. The MP boost is nice but utility wise its alot less and much more hindering. At higher levels you can Garuda - Blinkga everyone, however thats a hefty 100mp. Pre 50, i find this to be a useless sub.

WHM/BLM - This is a useful sub all around for every level from soloing to end-game. Warp, Tractor(great for Raising), and Drain/Aspir is good for soloing. Sleep is great too, as well as the ever important Escape. Elemental Seal is something not to be underestimated where even my Sleeps have an...i guess 85% chance of landing on the monster IT++. At level 40 you get Conserve MP, which has been proven to save more MP as long as you cast alot. Its random, but it helps out when you cast frequently. The rest are great solo.

WHM/BRD - Reletively useless past lvl 20 and before level 50. Past the Dunes, you be better off with another sub preferably RDM or BLM. Pre 20 Paeon is very useful. Post 50 you can do Ballad and Madrigal for the Melee. Its a viable combination as long as you can keep up your Haste cycles and buffs as well as Cure. Time your songs and it will be definetly fine. I level BRD up along with my BLM so post 50, i can help MP regen the mages and PLD/DRKs along with RDMs. Of course with a real BRD this is useless in every single way. But without one this is perfectly viable if you can keep up. Ballad is MP Regen X however many people that have MP.

WHM/RDM - The alternative to BLM. The only thing is you dont get Warp, Tractor or anything. The real boon only comes 1-39 where BLM or RDM sub doesnt matter a whole ton. Around 30-40 it might be better with Fast Cast, however you trade Elemental Seal for it, so it might not be that great. Once you reach 40 however, BLM's Conserve MP beats RDM. At level 66 if you need an extra Dispeller it might be good to sub RDM. Other than that, it is not wise to use in an exp party.

WHM/PLD - ...................solo lvl 1-10?