Well... all the obvious ones* have already been said it seems, so I'll just mention three oft-overlooked reasons:
1. Superb value for money
2. A massive community of enthusiastic users
3. Simplicity
* eg no viruses, no malware, more stable, very compatible, intuitive user interface, stunning design, 'it just works', integrated bundled software, fun to use, runs Windows, company clearly on the up, easy-to-upgrade hardware, retail stores based around strong customer service, all-in-one desktop design with small footprint (iMac), power bricks built in, easiest to use remote control ever, Front Row larger viewing mode, movement sensor in laptops prevents hard disk damage if dropped, magnetic latches, MagSafe power connector disconnects safely if pulled, beautiful dock that tucks away or animates, snappy navigation with the wow factor using Expose, dashboard widgets, upcoming features in Leopard (x10), upcoming products like iTV, well thought-out integration with iPod + iTunes, cool ads, in-built iSight webcams, easy to set up, easy to change languages, culture of continual innovation, culture of releasing people into creativity... and more.