First off, what device are you updating?
Every device has different issues with different iOS versions. For example, iOS 7 crashed a lot more often on the iPhone 5s than on the iPhone 5. With the iPhone 5, the code base did not change nearly as drastically as with the 5s, because the 5s had to have the code transitioned to 64-bit. With iOS 8, the update introduced probably the most drastic under-the-hood changes to date that affect all devices.
Then there's the issue of how far into the current iOS cycle you choose to go before updating. Someone using iOS 7.0 will have a very different experience from someone using 7.1.2. iOS 8 is not yet at its EOL point, but even now, the reliability of iOS 8.1.2 is a lot better than 8.0 (and on my 5s, I would say it's at least on par with 7.1.2). The final verdict on iOS 8 did not occur on the date of release -- it will happen when iOS 9 is announced and the EOL version of iOS 8 comes out. People who value stability above all else, should just wait for Apple to announce the next iOS version before they upgrade to the current version.
Even the vaunted OS X Snow Leopard, which is now held up as some gold standard for stability and performance(or moment after which Apple's QA supposedly into a nosedive), was riddled with bugs and application incompatibilities when it first came out. The EOL 10.6.8 version of Snow Leopard didn't come out until 20 months later.
With iOS, I think the pattern has shown that you're generally safe making the OS update if your device is one generation removed from its originating iOS version. Go two generations removed, then you're looking at some performance compromises in return for the new features and API support. Once you're three generations removed, then you need to think long and hard before making the update. This is why I kept my iPad 3 on iOS 6.1.3, and my daughter's iPad mini has stayed at iOS 7.1.2.