We just returned from a 10 day trek in Upper Mustang, followed by a 4 day section of the Poon Hill trek. Exciting Nepal was very flexible, adapting their itinerary to suit us. Subash was very quick to respond to our email inquiries when we were planning our trip. Basu (our guide) and Budha (our porter) were very professional and helpful.
Being on the Tibetan plateau, we were treated to beautiful clear skies and great views of the mountains, unlike the haze-obscured views further south around Poon hill and Pokhara.
The scenery in Mustang is simply stunning, with some areas comparable to the Grand Canyon. Combined with the culturally rich landscape (ancient walled villages, prayer walls, stupas, Tibetan Buddhist monasteries and mountain passes), Upper Mustang is very special, although the new roads will change the trekking experience.
I definitely recommend trekking with Exciting Nepal