It's a step back in time. IOM is of course most famous for the TT races, those two weeks at the start of June that are a mecca for all fans of full thrust motorcycle racing.
For the remaining 50 weeks of the year it is an unspoilt island that proudly goes it's own way.
The scenery is phenominal. Because of the tax breaks on the island your pound goes much further than on the mainland. There are some really breathtaking places like the Laxey Wheel, Castletown and Peel.
Just as the laws and customs are unique to the island, so is the weather. It can change drastically in an hour.
The inhabitants of the island ( the Manx ) are about as friendly as it gets in Great Britain, by comparison the Irish are downright 'stand offish'.
I would recommend hiring a car to see all of the island. Car hire on the island is very cheap. Don't go through one of the major rental companies, go with a local company when you get there. Slow driving is the order of the day on the island, even though the roads have no speed limits.
It's a take-your-time, laid back, relaxing, sort of place.