Following the traffic here, I think the following should be (if it isn't) a FAQ item. Nonetheless it bears repeating here:
FTDI conversion chips and drivers are extremely stable. Anything else, including Prolific, is a gamble with poor odds. It's that simple.
Background: On our sailboat, I do a number of serial/USB conversions. This converts (admittedly ancient) NMEA 183 serial data from instrumentation for a USB port on a laptop running a navigation program (Coastal Explorer - don't leave port without it). Initially I used Prolific-based convertes (what did I know?). Simply put, it became one non-stop headache. Data wouldn't come through, drivers stopped functioning, etc., etc. I switched to an FTDI-based 10-port serial/USB converter. The only changes to the system was to plug the DB-9 pigtails into the new converter and load the FTDI driver. End of problems, no crashes, full functionality. To clarify, one port services the full NMEA 183 data stream from the autopilot computer (speed through the water, depth, wind direction and speed, full GPS data stream [position, speed over the bottom, direction, GPS status, time of day)) and the port sends steering information to the autopilot, waypoint location data, and more. Another port sends data to the VHF and SSB radios (position, time of day) and receives data from the VHF (Digital Selective Calling data including emergency data received). Another port sends position, time of day, waypoint location data and receives target data from the radar including Modified Automatic Radar Plotting Aid data. The last active port receives data from the Automatic Identification System and an auxiliary GPS receiver (part of the AIS system). (full translation of "NMEA 183", "waypoint", etc. available off-list)
This is a long and probably very boring summary of what goes through FTDI devices at a mix of data rates (4800 baud for all but the AIS, which works at 38400 baud). If the FTDI part fails, life gets ugly fast (yes, there are backups). I bet a lot on FTDI and have no cause to regret it. Prolific devices were... my wife wore earplugs when the converters failed.
Again, FTDI good - not FTDI bad.
73 de KC3DOO Rick