I have an amplified classical guitar, and I'm thinking about buying the Behringer UCG102 guitar-to-USB interface.

However, my laptop has a USB 1.1 input. Do you think it would work well, or it supports only USB 2?

USB 1.1 is not only slow but a laptop that old to have that old a port is going to be even slower......

It will work, but you may have latency when you play.......in the end it's probably worth it to upgrade to a portable recorder or just use a proper audio interface and a faster desktop computer.