SCBWI is definitely worth it -- if you actually go to the conferences and events (even just your regional events). Anyone can join, so just being a member isn't going to necessarily benefit you in any way. However, if you actually go the events and network, it's VERY useful.
Many regions do allow you to attend their conferences and events even without being a member (but often charge you more, and members get first dibs). So, it may be worthwhile to find out if that's possible in your region. It's the networking and learning experiences that are valuable -- not the membership card.
I've been a member for a couple years and will always be a member, even after I've reached my publishing goals. SCBWI is that great, in my opinion.
ETA: Agents don't necessarily care that you're a member, as I said above. However, many agents with closed submissions speak at conferences and events, and they often open submissions to participants at those conferences. You can also meet one-on-one with agents and editors at many conferences, which can help get your foot in the door. I know several authors who met/got connected with their agents/editors at SCBWI conferences.