I was enhance from lvl 1-53 and had no problems with it (still dont). But I respecced to elemental just to try it out and in my opinion it is 100 times better than enhance. For leveling purposes up to 50, enhance is probably best, but after 50 i suggest elemental. With enhance (as said above), in instances u have to be close to mobs to deal your DPS and most times your group will wipe because the healer cant constantly heal you and the tank/tanks. Also, most enhance shammys dont stand a chance against a good caster. If you cant get close to your target then you are going to die. If you know where to put your talents in elemental and what gear to get, elemental is much better than enhance at higher levels.
Eg. When I switched to elemental, the same monsters that had been killing me when I was enhance only managed to get 1 hit on me if they were lucky.
The final thing: Elemental shammies dont wear cloth. This is the biggy! When some enhance shammys switch they believe they need cloth to make them better casters, DONT DO IT! The great thing about being an elemental shaman is that you can deal 50-90% of your damage from a distance and if your wearing mail+ a sheild you can finish the target of in close quarters. Lets see a mage or lock try to do that without ice barrier.
BTW your best friend against casters (ESPECIALLY MAGES!) is purge. Spam that maybe 3 thimes and beating the mage has never been easier. And resistance totems can save your ass against mages if you know what spec they are ^^
Bottom line: 1-50 Enhance is better 50+ Elemental/Resto is better.