Kramer make cables with in-built transmitters and receivers. Those cables work. Magenta Research make Excellent DVI extenders that are very flexible. They can be used with Cat 6 or Fibre and even daisy chained. However you need to know a little what you are doing, e.g use the adequate fibre or Cat 6 cable and be aware of how do deal with EEID data.
I have used the Gefen boxes over 300m without problems. Current I use Magenta Research. Usually the cable is not up to standard, there is noise in the system (no power filters/ conditioning) or the EEID data gets confused somewhere because either DVI detectives are not used or not correctly programmed.

Sven