I have only seen 2 in the Denver metro Area.
Not too uncommon.
I see about 5 a week on the LUL radar flying somewhere.
Depends what you mean by “rare”. Avantair is the largest user of them in the USA with a fleet of 57 aircraft. Denver is a frequently visited airport, so I would expect you to see more than two aircraft there in any given week.
There are also several privately owned P180s in the USA and CA.
Full disclosure, my son is a Capt. for Avantair.
Denver has at least two based in the metro area, neither are Avantair. Todd Helton, I believe, is an Avantair customer though.
We have 2 of em