I've had to "settle"

for the Diamond Supra Express USB, simply because there's no room for any more serial devices (or anything else for that matter) on my system. (OK, I kinda like the blue translucent case too

)
After a ton of testing (Yes, I'm a Diamond fan, but not on the payroll), the USB modems that fared the best in all testing I've done, are the Lucent chipset ones. I'm also a tried and true Rockwell/Conexant fan, but have to admit that if you MUST have a USB modem, the Lucent is the only way to go.
More here:
http://www.56k.com/ubb/Forum7/HTML/002430.html
More here:
http://www.pcmech.com/article.htm?great1