Been using TV out since before windows, when DOS was king.
Huge difference, with ATi as good as it will get, and nvidia as bad as it will get.
Colors will be more vivid.
Brightness and contrast will look great with ATi, and terrible with nvidia.
Little things like reliable sizing of screen that holds after reboots.
TV out is mandatory here, and all my nvidia motherboards have onboard TV out. Like obeseotron stated about nvidia's TV out: "the output from Xbox media center is light years better than (nvidia) TV out from a video card". All required adding a cheap ATi card to improve TV out, and they all now look as good or better than xbox video out. Motherboards in use here with ATi onboard are great.
Using TV out for HTPC, with Sat DVB-S tuners, as well as occasional internet browsing.
HTPC to TV has EPG (TV guide), and with ATi, the image quality is as good or better than any set top TV box from cable company or sat company, or others, including TIVO. ATi = Excellent TV out.
The TV out was the final deciding factor on new PCIe card, and choose ATi x800 over nvidia 6600GT. X800 looked great. 6600gt looked as bad as all prior nvidia cards for TV out. Saying Nasty was being nice.