A Slightly Different View
I was a memeber for a year and went to the big LA conferece, which was pretty awesome. I think the message boards are really similar to what you get on this site for free, and I can't say the membership did a whole lot more for me than giving me a discount on the conference rate.
I'd say join if you are going to attend one of their conferences very soon or are looking for a critique group and can't find one at this site.
I have to say that I've heard people in the industry say they are not terribly impressed when they see that a writer who's looking to be published is a member of SCBWI, so I wouldn't join if you're only joining for that reason (which I'm sure most people don't. I'm definately not implying that that's why Jimmer might have joined, so no offense meant).
Also, if you are already published in the children's literature genre or will be very soon, you might want to join so you can be eligible for SCBWI's Golden Kite Award.