Give it a miss!
We visited with my family in July 2005 and really regretted it! It cost $16/person for admission and was not worth it!
The tour involved an uninteresting walk past the Edison winter home (which was under rennovation so you couldn't go inside) and tiny Ford home. They didn't even give a tour of the Edison laboratory and I learned more history from the museum that adjoined the gift shop.
In my opinion, the best part of the experience was looking at the plants and trees on the estate which included an amazingly huge banyan tree (largest in the US, 2nd largest in the world) and the interesting trees (pond apple, mango, lychee, hanging sausage, bamboo) that were scattered through the estate. Aside from pointing out a few of the trees, the tour guide didn't cover these.
This attraction is historic, but is not run by the national park service! And the fee they charge is ridiculous. Unless you're a huge fan of Ford or Edison, I'd give this a miss!