This question about unions for IT people comes up about once a year on Slashdot. Every single time you see the same damn bullshit from people who have no fucking clue what a union is or how it works.
1) The members (workers) have to vote on the contract. Don't like it? Don't vote for it. And you don't pay any dues until the first contract is negotiated.
2) Think performance bonuses are a good idea? Fine. Keep 'em. It's your contract. You can make the contract read whatever you'd like.
3) All the contract is is a legally binding document that spells out the work rules so management can't arbitrarily change them. If they do break the rules, you've got a legally binding contract to back you up. Imagine, you can keep all the same rules and procedures you have in place now except they could actually be enforced.
Take deep breaths people. If unions get the support and input from their members, they can be one of the best ways to empower workers and and make the company a better and more profitable place.