Elgato has a number of Eye TV - tuner options that work fine. I've used it to grab Component and Composite signals with no problems. Downside is you can't control the cable box easily.
Another option is, if your box has firewire out enabled is to grab the signal directly from the firewire connection. Googling that will give you directions.
Not as elegant and definitely a hobbyist solution but if you have that option it is a free one.