If you are thinking about transferring professionally recorded music from tape to CD, consider hard. As Equill says, computers are internally noisy, so use an external device for best results. I've used the Griffin iMic (a USB device) and had to play with hubs, cables and connections to get satisfactory results.

Other downsides are one-to-one recording time, tape noise (you get used to it if you only listen to cassettes, but drives you mad otherwise) and track tagging. Tape to CD is hard work, and (having tried it for many hours) I would only use it for irreplaceable recordings.