I've owned more legit copies of various versions of Microsoft's flight-simulator over the years than any other paid for software I use. I'm no Bill Gates fan, its just that their flight-sim has always been a GREAT product and I'm picking it always will. The really killer thing about it is the open architecture which allows 3rd parties to create add-ons for the game, many of them free.
After playing FS out of the box you quickly tire of trying to use the Cessnas as sport-planes (which they're not) and get sick of crash landing 747's (coz they're so massive). When the desire to ruin other aircraft hits you, and it will, like some drug-addict, then head off to places like http://avsim.com and http://www.flightsim.com for free aircraft downloads painted in nearly any airline livery you can imagine. Its pretty easy to create your own skins for aircraft you like, check out My Repaints for example, all done using the Gimp.
If you're a kiwi you'll definitely wanna get along to the NZ FF forum site http://nzff.org/ and if you have the budget and a flash enough computer to run the latest version FSX then checkout http://www.flightsim.co.nz/ which has loads of NZ used aircraft in local liveries.
After playing about in a few new aircraft you'll definitely want to start downloading scenery packs for airports, cities and regions you fancy flying about in. The base scenery isn't bad but there's loads more detail in the add-ons and once you see some of the quality of the free and commercial stuff there'll be no goin back.
One of the final steps in the devolution of a flightsim junkie is joining a virtual airline. In NZ that would be Virtual Air New Zealand of course, based on the real world Air New Zealand. They currently operate a full flight schedule based up the flight schedules and timetables of the real world carrier using a full fleet of aircraft, as used by the real world carrier.
If you'd like a sample of what the Sim is capable of and want to see the sort of crates I've been blasting around in then check out my collection of Photos of flights in various stages. You might also have a squiz at some videos on youtube. Here's one I created recently to showcase some AI Jets sitting at Queenstown airport:
If you have the addictive personality gene, or have a busy career or family life, then steer well clear of simming. It will ruin you and those around you. If you'd like to read a little about the steady decay of a flight-simmer then check out my blog post on the 10 Signs Youre Addicted To Simming