How does ethereal compare with other protocol analyzers?
What features do commercial products have that open source products may lack, if any?
Is ethereal powerful enough to use in preparation of the CWSP/CWAP?
I would stay away from Ethereal if you ask me. I hate that analyzer. :-)
If you're studying on an extreme budget (as most are), then check out Tamosoft's CommView for Wi-Fi.
When installing, use the /CWNP switch, and you'll get 30 days of full use. This is the most cost-effective approach in my opinion, and if you don't fall in love with Commview for Wi-Fi within the 30 days, I'll eat my shoes. :-)
It's full-featured, very inexpensive, and worth every penny.
Just one thing to add Devin...
If you get Tamosoft's demo version from a link that is listed inside the CWAP book, you actually get a full 60 day trial of their analyzer. Not just 30 days. I'm currently using it on a trail period here at home. Stayed up till 1am last night in fact using it to find another station that's probing my AP.
Ethereal is more of a LAN analyzer. It lacks reporting, Wi-Fi protocol isolation, performance summary, graphing and alarm functions that you get in a top of the line product like Wildpackets Airopeek NX. Ethereal does filtering, but in my opinion commercial products make filtering a lot easier.
If you use Linux, Ethereal is OK because at least it does Wi-Fi captures. On Windows and OS X you have to capture in another application and open the frames in Ethereal because it has no drivers for putting Wi-Fi adapters in monitor mode.
As for Devin's comments about CommView for Wi-Fi, you can start eating your shoes any time :).