When using XP I've never had one fail, not even cheap ones or common brand ones.
When using Vista I'm still trying to work out the secret to making any work. I haven't tried Gary's and that would be next on my list if I give up with the 3 I have.
To control my 350D I have lashed out on a
DSUSB box and cable to completely remove RS232 from the equation.
To control my Losmandy I'm still stuck with RS232 converters. I've found recently that I can get them working, usually by forcing them to a com port like 1 or 2 which is in theory already in use, but have not had consistent success. So I agree with Monte, "quirky". I would suggest best avoided if possible.
Roger.