PDAs made by Palm or Handspring (now part of palm) are the only compadible PDAs for the Mac O/S. However for Sony, Compaq, etc there is a company out there, called MarcSoft or something (listed at apples compadibility list of isync).
I have a Sony Clie TJ37 and I love it- 200mhz (pretty fast for a palm based), Digital Camera (works ok), WiFi 802.11b, small, Free Documents to Go (all for $300!!). I took a look at the new Dell axims that came out...
One model has a 624mhz processor, WiFi, Bluetooth, portrait/landscape switchable display-Pretty nice- $350!!
I am not a huge fan of Compaq iPaqs- they seem over priced compared to their compedators. Other than that I still do not know why.
I like the Palm Based Sony software and the Pocket PC software more than the basic Palm software. The sony and the p.pc seem a lot more advanced. The new Palm Zire 72 looks nice with its video recording camera and stuff adn the zire 31 with color screen
Good Luck