Usually, those aren’t RS232, but so-called TTL-level serial ports: this means that they use diferent voltage levels (0 – +5v vs. -12 – +12v), and you need an adapter to connect them to your computer.
The USB-BUB II it’s based around the FT232R chip: it connects to an usb port, and provides a TTL serial, complete with DTR and CTS signals. But the board has a couple of handy – and unusual – features: first of all there’s a polyfuse, to protect the USB line from short circuits. Also, two smd LEDs – one green and one red – that shows activity on the RX and TX line.
The board can also deliver 3.3v or 5v power – but don’t count too much on that, the maximum current is 50mA. In case you need some power, however, on the back of the board there’s also space to solder an external voltage regulator.
The BUB use is straightforward: drivers are available for Windows, OS X and Windows CE (!), and recent versions of Linux kernel supports it natively. Also, the board is delivered fully mounted, you just need to solder the header, or the wires, and connect the usb. It works pretty well at all the speeds I tried, and it doesn’t suffer too much from interferences, from what I can say.
Overall, a pretty useful device – a bit pricey (€13,50, shipped in the EU, from The JeeLabs Shop), but still a good choice.