I had used the Keyspan USA-19HS exclusively until I ran across one particular radio modem that didn't like it. For that device, I have started using a cheap one I found in a box of old parts. I have used several of those offered by GIT for A/B stuff and they look like this one:
http://www.plccable.com/usb-to-rs232-plc-programming-cable-special-plctalk-net/
I may order a couple of those since just Friday I learned that our new guy doesn't have a plain USB-RS232 converter in his kit.
I would recommend getting more than one type if you are in a field where you never know what sort of gadget you might need to communicate with on any given day.