If you're having trouble with your device you may have an incompatible driver installed on your machine. If you're using a device based on the Prolific PL2303 chip, the chances of this are high.
There are a few dozen different drivers available for the PL2303 series of devices, all having varying levels of compatibility with different Mac OS X versions and certain devices themselves. One driver may work fine with one device, but fail with another, even if they're both using the same chip. To make matters worse, the manufacturer has not updated their driver in years, and instead instructs users how to disable important security protections on their Mac in order to get their outdated driver to load.
If you're using an incompatible driver, the most common symptom you'll experience is the device seems to open okay, but no data can be sent or received. Other, less common problems increase in severity all the way up to a full-blown kernel panic, which crashes your entire computer.
The solution is to remove these drivers and allow Serial's built-in support for these devices to work. When a driver is available for a device, the driver takes precedence and Serial's built-in support cannot be used. The distinction is normally transparent to you, though you can check to see which driver is in use by choosing "Device Info" under the Settings panel.
To remove a troublesome driver, you'll first need to locate it. Serial will show the driver's "bundle identifier" - the unique string used to identify programs on the Mac - under the "Device Info" panel. Once you have this, you can use the following command within Terminal.app to see where it is on your disk:
% kextfind -b bundle-identifier-here
They will be located under /System/Library/Extensions or /Library/Extensions.
BE VERY CAREFUL WHILE WORKING IN THESE DIRECTORIES, AND DO NOT REMOVE ANYTHING THAT MENTIONS APPLE IN THE NAME OR BUNDLE IDENTIFIER. YOU CAN RENDER YOUR SYSTEM INOPERABLE BY REMOVING THE WRONG DRIVER.
Next, make sure the device in question is unplugged from your USB port before continuing.
Then, within Terminal.app, move the driver from where it currently is somewhere else on your machine- your desktop, for example:
% sudo mv driver-path-here ~/Desktop/
This will move the driver to your Desktop.
Finally, reboot your machine.