[Updated 11:26 EDT]
It has only a few new features, most notable of which is Siri on the Mac.
Siri works ok, not great. And even when she is fully understanding my requests, there is only so much she can do. Great for looking up faces in your photos, good for looking up things on the web, not so great for making phone calls, and why would I want her to open Word when I can do so myself in 1/2 the time?
A perhaps more important feature is the ability to copy and paste from the Mac to the phone. That is very cool — not needed often, but nice when you do need it.
There is better file sync’ing through iCloud (for files on the desktop or the documents folder), There are some file space optimization features (it can be set to delete videos you’ve watched) and everyone’s favorite useless feature: bigger emojis!
A really nice, if small feature is that clicking on the speaker in the menu bar allows you not only to set the volume but to switch among your output options. Excellent
So far, no negative effects on Xamarin Studio or other programming tools.
If you are being too productive, you’ll love the picture in picture feature, which lets you watch a pop out video in a small window while trying to debug your program.
Oh! yes, I almost forgot, you can unlock your Mac from your watch. Why would you want to? I have no idea. And it works from 5-9 feet according to CNET, which means you can “lock” your phone but if you’re 5′ away, it won’t be locked at all.
It broke my Evernote scanner, so I had to plug the scanner into a non-upgraded laptop (grrr). This apparently is a known problem with this particular scanner, however, so you should be fine with printers.
It does not support my Bose Companion 5 speakers and there is no word from Bose on this. Very annoying.
On the other hand, the upgrade is easy and free, but I’m not sure I’d rush.