I've found no degradation through the DVI switching, tested with both HD cable and an OPPO DVD player.

The DVI standard limits cable run to 18 feet, so a 30 foot cable violates the spec. There are some manufacturers that claim to be able to build a 30' cable that will work, and there are also some signal boosters that will extend the range of the DVI signal. I haven't tried either, however, since the longest DVI cable in my system is around 4'.