Just today downloaded and installed Windows 10 on My Lumia 720. Done with using insider app which is availiable in Store. It took much time for me. Firstly when I tried to update with Windows 10, I had to uninstall WhatsApp which was occupying around 1.7GB space and the free memory was around 700Mb, while the required was around 2 Gb.
Everything goes well until downloading, while installing the two gears appeared and then a Sad Smiley and I was sure that I have lost all the data in it, Thanks to Cloud Services I retained the important things.
Then Googled what to do, tried Hard reset but no Luck. Only option was using Recovery tool. Lumia Recovery Tool helped me in Flashing the Windows 8.1 software on to my Lumia 720. It took around an Hour.
After this I said myself, lets do the upgrading thing again and downloaded Insider app and started again. This time the device successfully got the Windows 10 and it took around 2-3 hours for all the set up on the device.
So I am now checking out the new features and enjoying the cool OS windows 10. The work done is quit good and I am happy to see the things around in my Mobile. Will surely post the screenshots and videos af the new Windows 10 insider preview OS version.