This Pokemon has one of the most interesting Pokemon Powers that exists. Clairvoyance lets you see your opponent's hand! If you know your opponent's hand you can know when they are going to do something or if they have something like a Double Colorless Energy or PlusPower.
Water Gun
Water Gun won't have much effect in today's enviroment. Maybe unless a fire deck, but most likely its a attack that's just gonna get yourself KO'ed.
Huge Weaknesses
This Pokemon has 3 Huge Weaknesses that really cannot be taken care of. One is having a low HP of 40 which is pretty much vulnerable to anything. The second is that it is Weak to Grass. Making Scyther killing it in one hit no problem. The third is evoling from Mysterious Fossil.
Mysterious Fossil
Another real bad thing about Omanyte is that you have to evolve it from a Mysterious Fossil. MF are usually targeted and Knocked Out (without a prize, of course).
Combos
Besides Omanytes downsides he does have some Combos. Use him with Rocket's Sneak Attack, Rocket's Trap, Imposter Professor Oak's Revenge, Energy Removal and Super Energy Removal to completely destroy your opponent to nothing.
Rating
I would have to give Omanyte a 5.5. He has those 3 Major Weakness's which can't be covered and his attacks stinks. He got that high cause of Clairvoyance.
