Who has them and what did it do for you?
Just added a x cross over pipe on my 2.5 dual exhaust with Doug Thorley headers(460). It took away that blap,blap sound at idle,it's a little quieter and way smoother at mid range and wide open.I'd say that this is a must do,2.5" X pipe from ebay for 26$ shipped and 80$ for the install at the muffler shop.Google it on the purpose and benefit.

 

Its always been my understanding that high RPM/high compression prefer "X" pipes (HP Gain, better cylinder exhaust evac) while Low RPM/Low compression prefer "H" pipes (Torque gain, better exhaust expansion).

 

My opinion is it depends on the type of muffler used. Chambered should use an h and straight through should use an X