I bought one of those 18 +'s to use with my PF3000.  I had a problem where I could not make that unit not give the PF3000 too much data.

The unit is sitting on the shelf waiting for me to have time to set it up and use on my Ipaq and farmworks.  I do not think it will be a problem to work with guidemate.  The only problem I see, in my limited knowledge of the brains behind the GPS units, is that the receiver is not as "smart" as the more expensive ones.  It will give out the raw data but will Guidemate be able to handle it correctly and show the best guidance information from it. 

I plan to get it hooked up and use it on the ATV for creating new field lines and measuring fields.  Hopefully I can get it put together this winter and can give a report.