Pros: lighter thank a comparable metal tank, if your making your own you can make it exactly the shape you want.
Cons: Like Bob said, fiberglass or carbon fiber don't like to bend, however, if you're making you're own you can put a layer or two of kevlar in the mix. Also, there are some concerns about fuel eating away at the resin, but it can be alleviated by using certain resins and also sealing the tank.