That is the question. For many years, the debate regarding whether an older version of your iPhone should get an update has raged many times. Opinions vary wildly, and each preference is usually sufficiently backed up by statistical data or, at least, by a solid user experience. But instead of the decision itself, let’s briefly try to look into what, exactly, causes users to make these decisions, the bugs/fixes, and the version’s advertised features, and see if the update fares well overall.
Any software update is accompanied by bugs along with the fixes a previous version might require. For iPhones, however, this can either mean a simple annoyance or a major inconvenience. This was what displeased so many users as persistent Wi-Fi connection issues were fixed. Sometimes, only some devices get the fix, while others get something worse. In fact, this notorious “phenomenon” has been so sensational in the media that the original name of the fix stuck: WiFried. And yes, it still persists on a somewhat limited level in iOS 8.4.
So, what’s to be observed from this? The decision to update your iOS version will be primarily based on whether or not there is directly something you need fixed or repaired on your previous system. If everything is working fine using the previous version, then you might want to look into other users who adopted the changes of the new update first. See if there would be more or less no problems to the version of the iPhone or iPad that you have. If you need something fixed, also check if the new version directly addressed the problem although at this point, if you did not see anything mentioned about it in particular, the decision to update or not becomes a matter of pure luck.
Updates are also accompanied by updated or added features, and depending on how you use your unit, these may be essential or just pure garbage. However, having bloatware is just one part of the problem. Updates may also render entire features from the previous version unusable or incompatible with the new system. iOS 8.4, for example, has recently introduced a new featur named Apple Music, which kind of annoyed a considerable number of users with its compulsory nature. What’s worse is that one very critical feature that was already long available suddenly disappeared, the Home Sharing Audio feature. It might just be a coincidence, but the timing of the implementation and disappearance of both features made the 8.4 update suddenly a lot less appealing than it should be.
The decision-making for this one is pretty straightforward, so long as other feature updates for the new version do not outnumber the conveniences of the features of the previous version; if this is the case, then there is no direct or urgent need for an upgrade; at least, not for the next few 0.X’s.
As a rule of thumb, try to check if a considerably older version of an iPhone or iPad (think of somewhere around 4s or the new iPad) could still function relatively well with the new updates. That would give you a good idea if the new updated features were mainly beneficial to your iPhone.
On an added note, the subject matter about issues, regarding upgrading old hardware to new software is, more or less, no longer an issue for the latest versions of the iPhone and iPad. Unless the unit would really cripple itself with its own hardware limitations, then, spec-wise, an update would, most likely, still provide something positive for your device.