I'm a lvl 52 Enhancement shaman and Have played with all three tiers.  Just like any Class you have different builds for different things.  For lvling I choose Enhancement.  I did a few lvls as elemental and liked it alot but I prefer just throwing WF on a nice 2hander and going to town on mobs.  But if you want to know the truth I wouldnt PvP as a enhancement shaman.  One cause I prefer the range of a Elementalist with crits from Bolts and chains and you can absolutly rape any class if you get the drop on them.  I mean it sucks for them when you are behind them and hit your chain lightning from max range and crit off it and follow by a bolt for 60% less mana...then you drop a earthbind and frost shock build distance and continue the bolts...you can rape most none caster classes in a matter of about 5 spells and if Lighting overload procs forget it there nothing but burnt to a crisp.  But from my first comment it is all based on what your doing with a shaman.  A shaman is a balanced class.  Not a master of pure combat or pure spell casters.  But let me put it this way to you.  Each patch they tamper with builds so you got to respec constently.  Elemental is a soild build and so is resto.  No one wants a enhancement shamans in end game cause if you are doing that amazing DPS you have to be next to the mob and when the mob lets loose with there AoE and you die cause the healers are busy healing the main tank you have just sealed the fate of the group cause they lost a person and his totems.  SO solo/grinding/ Balanced= enhancement........ End game raiding, PvP= Elemental....ENd game raiding, instances= resto.