Well its a mixed bag, but a friend of mine has played a bit on the china server, and he has a few notes...some that have been echoed on these forums.

Lightning: Hes great early game, but when 60 rolls around you SHOULD swap out...he falls off and hard.

Earth: Hes not to hot on our servers right now because a lot of the higher end earth shinobi haven't been released yet, so his team setups are kinda in limbo right now. But remember using the earth main means you dont need to get a tank, and you only need to get dps/support nins and that can be worth its weight in gold.

Wind: Again, the later the game goes the better she gets. You can easily make her a full support main, and again free up formation stuff later in the game, not to mention the dreaded " Empress" Combo which c*l be gotten in stores, and at this stage of the game can be hilariously good.

Fire: Hes just all around good, and anyone that can neuter healing will never be " Weak". Sadly he never played much fire, so he hasnt much tips here.

Water: Late..late bloomer. Id REALLY suggest playing a diff class to 60 than swapping, because the water grind can be real.