It depends on what you do and how often you work with others. If you work with others you should match what they have. Microsoft Office for example will display the same PowerPoint presentation differently on each machine.
It also has to do with how much money you have to spend. An Apple product will cost you more money upfront, compared to a similarly build Windows PC.
Do you want to upgrade in the future? Apple products are more locked down when it comes to upgrades. If you're talking a laptop you're pretty much limited to hard drive and memory. Apple has taken to soldering the memory in and you have to purchase as much as you need when you buy the machine.
What do you have now? Odds are you will have to repurchase any software you're currently using if you switch.