I installed iOS on both my iPad Retina (3rd gen) and my iPhone 4s. I had some misgivings, since they are both at or near the bottom of the barrel in terms of supported hardware for this release. After some research though, it sounded like the iPad would do reasonably well. As for the iPhone, well, I should have a newer model in a week or two if it doesn’t work out.
My report is that both of them seem to work just fine. There may be a few more rough spots and hitches than before, but if there are, they are rare and subtle (for the most part), and they are counterbalanced by new features and refinements.
One issue on the iPhone 4s is that all the other models supported by iOS have taller displays, and so some UI is more suited to larger phones. The only place where this really stands out is when composing an email. Between the predictive text selections above the keyboard (which can be hidden, if desired) and the new UI affordance for easily switching between the email you are drafting and other messages, there isn’t much room to read the text you are writing.
There are definitely signs of memory pressure though. Tabs in Safari need to reload more (too) often and switching between some apps can take a little too long.
My advice for iPhone 4s users is to wait until they release a round of bug fixes, unless you absolutely need to try the latest and greatest.
As for the iPad, I don’t notice many differences in interaction experience from iOS 7. Safari may need to reload tabs a bit more often, but no where near as often as on the iPhone.
My advice for anyone with a 3rd generation iPad, or better is: If you want it, go for it.
If you want to know why you’d want to upgrade in the first place, there are lots of reviews of what’s new in iOS to choose from. Its worth noting though, that some features aren’t available on all devices. It looks like my devices will be left out of some of the seamless handoff as you move from a Mac to your iPad, or vice versa. One thing that just worked though, incoming phone calls show up on my iPad, and I can choose to answer them there, if I want. Not a huge deal, but convenient in some situations.