For more than a week I've tested 3 different software for syncing Outlook with my iPhone 6: gSync, Akruto and Companionlink.
gSync works through putting Outlook data to your google account and then you could sync it with your iPhone. The good thing about this software is that it's the cheapest option ($20), the bad thing is that it keeps data in the Google cloud. This was the main reason I gave up with testing this software.
Akruto ($35) can work via Wi-Fi or over the Internet (without using cloud). I've tested it for 3 days and seems that it works great with transferring Outlook data to all iPhones (tested with iPhone 6 and 4s). The good thing is that you don't need any extra app on your phone to work with Outlook data. Setting it up to work via WiFi took only 5 minutes comparing to half an hour with gSync.
Companionlink ($50) works through USB, Cloud and WiFi. Only tested its work with WiFi, but results were disappointing. The software created tons of entries of a single appointment, so it took an hour to clean up this mess. Also, in order to see and manage your Outlook data from your iPhone you have to install 3rd party app called DejaOffice. It's a pain when you need to switch between this app and your native phone's apps.
So, at the end of my trial period with each software, I will probably stay with Akruto. Here's how it works with iPhone if somebody will have the same problems:
http://www.akruto.com/sync-iphone-with-outlook/