Yes, it is a country - it has it's own stamps, it's own bank notes, and it's own parliament. It's a country Full Stop.
I popped over to the Isle of Man for a long weekend in March, and it's a lovely place, but if I were to offer you
one piece of advice it would be, go after Easter because then most of the tourist attractions are open! Having said that I did enjoy my trip enormously. Life is a bit more laid back, this became apparent even before I arrived on the island while booking my hire car, I offered to give the company my credit card number to secure the booking, but they said it wasn't necessary.
The island isn't that big, and you can travel from top to bottom in a couple of hours. Castletown in the south is a beautiful little town, with a pleasant town square over looked by a biggish castle. It was in Castletown that I spotted my first (and only) Manx cat. He popped out from under a bench, allowed me to take photo, refused any payment, and then disappeared under the bench again!
Also the Isle of Man was my 70th country.