I have tried XVCD compatibility with one XVCD's downloaded from VCDHelp on the 10:th of July 2002:
I have tested this on the United 3155 and there were no problems to play this XVCD disc!
I have also made an XVCD myself with four short movies (352x288 resolution) in different bitrates:
As far as I could tell, the 3155 played all these movies without any problems!
For more information about some XVCD tests on the 3155, look at the "Official VCDHelp.com PAL/NTSC Demo Video-CD" test results at the VCD compatibility test page.