Usb-serial converters are very hit and miss, sometimes you get lucky sometimes not, depends on your individual application and chipset that the adaptor is based on, either individually or in combination. The ones we have found most reliable at work are based on "Prolific" usb-serial chipset, but you have no way of knowing what's inside them generally.
PCMCIA RS232 converters are much more reliable, I haven't come across a situation yet that they haven't worked.
I recently bought this one - http://cgi.ebay.co.uk/PCMCIA-To-RS23...713.m153.l1262 and using it with Emerald with no problems. I'm not particularly reccomending this one, it was one I randomly chose, but just confirming it works with our K3 emerald.