Most of the common dirt-cheap USB to serial adapters are based off the prolific chips (FTDI being more common when you need multiple ports, extra GPIO, or alternate serial protocols like SPI/JTAG). They work just fine. What is often total crap are the driver CDs that no-name manufacturers ship with them. They work perfectly in Linux (good drivers included in the kernel) and well enough in windows if you download the reference driver from prolific. The PL2303 doesn't have unique device IDs like the FTDI chips, so they can't support persistent device names (i.e., always enumerate as COM6).
Many people will warn you that some of these devices have TTL logic levels instead of the standard +/- ~9V RS232 levels. They do this to save the cost of the charge pump to generate the negative voltage from the +5V USB. This is horrible and wrong. I have personally never seen such a device except for something like a dedicated PDA adapter where presumably the manufacturer knows it works with the device in question. Still, it pays to watch out for it if you can't figure out why something doesn't work.