Some people have complained about cracking. But nobody I've met has had any trouble. Think about how long they've been selling plastic laptops, and how many other manufacturers still use plastic. In fact, loads of stuff you use every day is probably made from plastic. If cracking was some huge epidemic, don't you think everyone would have stopped using it?
A few people on the internet have reported problems with the plastic. A few people on the internet have reported problems with the unibody MacBooks construction: there were incidents of loose battery covers, crooked keys, etc. Are you going to not buy one of them because some people have reported that problem?
The plastic MacBook is a phenomenal machine (remember, same specs as the unibody MacBook) and is very well priced. If price is a concern for you, it's really a no-brainer.
Great value in the unibody MBs in the refurb store. But no Firewire means unless the OP has a HDD or SD based video camera, it's no good since they plan to run Final Cut on it. I would buy the new MacBook tomorrow if it had Firewire! :/