This is the first update of iOS that has downsides to it, namely the new maps app, but it also has a few really nice new features.
Maps
This will take some time until the data is even remotely a match for what Google provides. Search works badly, it does not find most businesses that you would expect unless you know the exact name, and when it finds some options, it points you 50 km away, where Google finds a couple of options within 5 km. Until now I never realised how closely mapping is related to search, and I can only hope that Apple will improve on that quickly.
I miss the terrain view, and the coverage of satellite images still has some way to go to cover all parts of Germany. They are also of a lower quality than those from Google, often having unnatural colours or showing cloud cover. Fortunately, Google Maps works reasonably well even in Safari, so there is a fall back when Apple is letting me down again.
I like the vector display mostly, it is a clear display, but the vectors are often too straight. It would look much better if the roads were modelled as spline curves. What I really miss is any indication of built up areas, this is important for driving, to know when you have to slow down, and it just gives a much better sense of location.
Routing integration could be better. If you have Navigon on the unit, you can only use it with some extra screen tabs, as a “public transport” provider, unfortunately it cannot be set as your default instead of the built in navigation.
When I checked the turn by turn navigation, it was reasonably efficient in the data transfer department, needing 500 KByte of cellular data for a 2 hour trip over 200km. I believe it could do a better job of displaying traffic problems, and I really missed the ability to easily get updates on alternative routes, but otherwise it already works remarkably well.
Single App Mode
Single App Mode is a great addition when you want to pass the unit to someone else to play with. While active, in-app purchases are disabled. The retries when trying to evade the sand box are time limited, with a maximum of 3 minutes. This would mean that you can break out after 20 days, which sounds like a reasonable compromise.
Bluetooth 4.0
This is a great addition, as it can be used without any extra license fees, and so would enable almost all NFC scenarios given its low power nature. It is also a way for lot of accessories to drop hacks like using the audio port to use the smartphone or iPad as their controller, now that the cost to add Bluetooth has fallen below 2$. One of these chips is the nRF8001 by Nordic Semiconductor.
The hardware is on the 4S, 5, the new iPod Touch (5th gen) and the new iPad, as well as the newest Macs. That is easily enough support to start using it.
Small Stuff
My favorite little improvement is that you can now update apps without being dumped out of the update page. There are also a lot of other small refinements to make your life easier, but unfortunately the camera connection kit still does not play nice with the Fuji X100. The German keyboard now has a variant with umlauts, but it is too tight to be usable on the iPhone, and I am afraid that using it will get me into trouble with my muscle memory.
Upgrade or not
The question whether to upgrade really comes down to your reliance on Maps. I was surprised when my mother asked me whether she should be holding off of upgrading because of it, and it shows how utterly important maps are to a mobile device. Currently Apple is far off from having good enough data in many parts of Europe. Satellite images are very often not available or of a too limited quality, the point of interest database is smaller, and it seems you can only find them as long as you know their official names, not by what they offer. Fortunately you can still access Google Maps via the browser, but it is more hassle to use than the old app, and you loose the ability to place pins on the map.