I have several Lexar cards and they work just fine. As for Sandisk, they have known troubles with the Axim; start with this thread:
http://www.aximsite.com/boards/showt...&threadid=4031, for instance. Some members have also reported problems with Lexar (and other) brands made in China rather than in Japan, though; you can find some more discussion of that in this thread:
http://www.aximsite.com/boards/showt...hreadid=143618.