There's not a whole lot of discussion here about this certification so I thought I might drop my few cents around. I passed this exam today after ordering the official study guide a couple of months back as well as the prep guide last month. Both were quite helpful. I hadn't actually given serious thought to pursuing the certification until this month. I've been using Ethereal / Wireshark for some years now so it's not like I went into studying for the WCNA cold, but I definitely did pick up some extra juicy nuggets of info here and there. I've also been conducting training sessions at work on this same topic so the reinforcement helped.
In short, if you know your protocol basics (IP, TCP, UDP, ICMP, and common ones like DNS, DHCP, ARP, HTTP, etc.) you should have a good majority of the exam covered. You don't need to be able to recite the RFCs, but in addition to knowing Wireshark as a tool you'll need a solid understanding of how everyday protocols work at a general level. The two books are very useful and if you're relatively new to protocol analysis, I highly recommend them, especially for the WCNA exam.
(Note that the official Wireshark study guide has a few errors here and there, so if you're reading through it be sure to check the errata:
http://www.wiresharkbook.com/updates.html)
Now that said, I didn't ace the exam. I did manage to pass it in 30 minutes, but it's not like I easily knocked down every bowling pin down the alley. I think the exam was pretty fair and you need to be detail-oriented (after all, it's protocol analysis) because when reviewing a network trace, one flipped bit in a field can make all the difference.
In general, it's my belief that being able to read and analyze network traffic is an extremely valuable skill. Sure, a lot of people can get by without it, but even if you're doing desktop support, I think it provides immediate evidence of something not behaving the way it should and you most likely will be able to point the finger directly to the offending host / router / user / application rather than making vague assessments like, "Seems to work after rebooting." This is especially true for the folks on the networking track (CCNA, etc.). While the CCNA may introduce you to subnetting, addressing, switching, routing, etc., it does not get into the protocols themselves that much. If you're serious about being a good network engineer, you should at least be able to comb through a trace file pretty competently. It'll put things in perspective that much better as you develop your career.
I didn't really pursue the WCNA for its marketing qualities in general. While it adds another four letters to my resume, practically no one knows what a WCNA is (yet, at least). Someone will probably guess that I misspelled CWNA. If you're going the security route, consider this cert as an opportunity to develop your ProtocolAnalysis-Fu because interpreting bits on the wire is an important part of the job.
