First of all, you don’t need to upgrade. You can get the free upgrade by clicking the Windows 10 logo in the lower right of your task bar to download a copy, but just because you download it, does not mean you have to install it.
An operating system like Windows is a major undertaking and first releases always suffer from stability issues. Windows 10 will be no different. Microsoft made early versions available to its development partners, but that does not guarantee that your current applications and devices will work on Windows 10. In particular, if you are running older versions of your applications, they may never work natively on Windows 10 (they can still be executed on Windows 10 via virtual machines, but that is the subject of another blog). You might be inclined to upgrade to the latest versions of your applications, but in so doing you might either lose features that you are dependent on or face a significant learning curve if the user interface has changed dramatically. Windows 10 itself will be different than its predecessors and it too might present a learning challenge.
By all accounts, Windows 10 is going to be an excellent operating system (especially when compared to Windows 8 and Windows 8.1). But, my recommendation is that you do NOT upgrade immediately. Give it at least 6 months to mature, and then evaluate carefully whether your applications and devices will function as you wish on Windows 10 and whether you are ready to tackle something that is different than what you have grown used to.