I have used the javax.comm api to communicate with a GPS receiver using
a USB-to-serial adapter on a PC running windows and have not had any
problems.  The only thing I noticed is that the port number is variable
so you need to programmatically determine which com port the
usb-to-serial device is registered on.  For example, if you plug in the
adapter it might register as com5.  If you unplug it and plug it back
into the same USB port, it would register as com6 (at least this is the
sort of behavior that I saw).  I dealt with this programmatically by
enumerating over the available serial ports and checking if the device
on the current port was the device of interest.

I also used rxtx to do communications when running linux and did not
have any problems using an adapter.

Good luck,
p