Well we need to get used to it — new operating system every 12 to 18 months, a little bit like with our iOS devices. This is all good as long as you keep your computer hardware up to date in a similar style to your iPhone. Well, not quite as regular updating required.
But although the App Store on your Mac will prompt you to update to Sierra that doesn't mean that your trusty Mac will like running it.
The exceptions, machines that trancend age, are any of the Solid State Hard Drive (SSD) computers. That is any MacBook Air, later MacBook Pro Retina Display and any iMac with a Fusion drive. These are fine.
Then there is a small matter of RAM, or Random Access Memory which isn't storage, the amount of space on your computer hard drive, but is working memory or how much the computer can do at once without getting the dreaded spinning wheel. Older computers ie before i3 processors need plenty of RAM to make up for their older processor so factor in a RAM upgrade to a minimum of 8GB. Cost is usually between $150 and $250 depending on the machine and how much RAM. You might find that your computer is too old to spend that sort of money on. Its a question of decisions, decisions.
What do we get in Sierra? Lots of new features including Siri for Mac so you can talk to your Mac and order it to do things!
Here are a couple of articles to get a taste. And as always email me if you're not sure...