TWC's digital converter is probably a simplified STB that will get SD versions of the locals and standard cable channels. It probably won't get premium channels, SDV, VOD or PPV. And no interactive guide. IOW, pretty much what your analog TV gets now. Might as well get one or more while they're free. Of course TWC might decide to start charging a monthly fee for them eventually.
Your last line sounded like you're thinking of buying a converter elsewhere. If you mean the digital converters that were briefly popular because there was a $40 coupon for them, they are strictly for OTA and won't work on cable at all.
This is Time Warner screwing the pooch. These digital boxes have been so problematic - Google the digital conversion in Maine! The right way to do the digital conversion is to just send them out as QAM channels so customers can just hook up the wire to the back of the TV like they do now. They should also eliminate SDV, as going all digital will free up that much bandwidth.