Has anyone ever used Mr. Nga Wang for touring Lhasa & Tibet?

No idea who he is, but in general most travelers with interest in Tibetan culture prefer Tibetan guides to Chinese ones. Wang is a Chinese name.

Mr. Wang actually lives in Lhasa.

I am sure he lives in Lhasa. But that is a Chinese last name, but Tibetan one.

Unfortunately a great many Chinese now live in Lhasa...

In the 2000 census, it's about 2/3 Tibetan, 1/3 Chinese in central Lhasa, officially the Chengguan District. I say in 2010, at least 2/3 are Chinese, probably more.

I do not know Mr. Wang. I can, though, highly recommend a Tibetan guide named Lum Bum in Lhasa. I spent six weeks in Tibet last year with Lum Bum. He and his wife Namdori run Tibetan Shaman Tour in Lhasa. Phil Borges (famous photographer of Tibet) mentions working with Lum Bum in his blog (see entry: How a Guide Makes for a Successful Trip).

I have to add Nga is I in Tibetan and Wang could be meaning Power. The problem i was told by a guide is to do business they have to have a Chinese name for a Bank account, business license. So Id ask them if they are Tibetan before judging how many Tibetans do they hire. The last one is very telling. Its usually 1% or 80%. Lumbum is great but make sure he goes according to your schedule, make that clear , not the other way around.