Nextags is an extremely old tool which came from the Microsoft Mobile Information Server install CD.  Yes, that's from over a decade ago and it was for ancient ActiveSync logging.  However, there are a couple of other tools which might help. 

I wrote a tool which could be used for EAS 2.5 and later called “EAS Inspector for Fiddler”.  It will convert EAS traces taken by Fiddler and convert the WBXML into XML inside of fiddler.  The full source and executable are checked-in.  The nice thing about Fiddler is that it acts like a proxy for SSL – i.e. you can set it to automatically decrypt the SSL traffic.

EAS Inspector for Fiddler
http://easinspectorforfiddler.codeplex.com/

Another app which may be of interest is EASTester.  This tool has multiple features.  If you export the WBXML for a call from a network trace (Netmon/Fiddler/Wireshark/etc) to a file and it’s not SSL encoded, then this took can read that file and display it as XML.  It can also convert WBML in hex byte strings into XML.  Finally, it has a window for submitting EAS calls as XML and it will WBXML encode it, submit it to Exchange and decode the response into XML.

EASTester
http://eastester.codeplex.com/

Both of these apps use code from the examples in the Exchange Server Interoperability Guidance documents.  Both tools should handle EAS traffic starting with 2.5 and later. Our own support engineers have been using these for EAS cases with some very positive results.  I hope that these tools will be of help.