Having an experienced tour guide is an amazing experience. Along with just knowing where the crowds will be (and won't) and helping with a touring plan, a tour guide is also your insider to history and stories about the parks. Plus, a good tour guide will also act as your personal assistant while touring - taking family photos with your camera, standing in line for snacks, etc.
That said, it's not a cheap experience, whether you use and independent tour guide or one of Disney's official guides. But if you have the means, or are looking to do something extra special (whether your first trip or a repeat vacation), it can definitely make a good vacation an amazing one.
Short of hiring your own guide, definitely pick up one of the guide books. I personally like the Unofficial Guide, because of it's completeness. I also recommend this website, especially the forums, because you can ask questions and get answers and opinions. I also suggest AllEars.net, because they keep things updated pretty well and have a wealth of info as well.