I've watched some of the mid to later seasons of TNG upscaled to 720p and 1080i.
720p is OK. It improves the image for viewing on a large LCD or plasma, and there's just about enough on the original for it to stand up to upscaling without any issues.
But 1080i was a different kettle of fish. Quite a few what I would describe as little black lines appearing on the screen where there presumably wasn't enough quality in the source material for it to cope with that level of upscaling.