Pal Mickey can provide you with information that even the most established Disney historians might not know about. A lot of the information he provides is very interesting.
One downside is that he can't be updated the next time you visit WDW - you'll have to buy a new one or have potentially outdated information. For instance, if Toy Story Mania isn't open when you visit the first time, he won't provide you any information about it because he doesn't have any. When you visit the next year, even if Toy Story Mania is open, he still won't have any information about it because he wasn't originally programmed with it.
With that being said, it is up to you to decide if you think he's a good deal or not. Yes, he does provide some great information, and yes, he is fun, but is it really worth $70 for something that might be outdated the next time you return to the parks?