Whilst Windows 10 has passed through the starting gate (shipping is probably the wrong term, as it would imply completeness) Windows 10 Mobile is still very much in the testing phase, with a launch planned for some time later this year.
Microsoft has some work to do before then, because based on my use of the current build on a test Lumia 735 the software is both brilliantly conceived and unusably laggy both at the same time.
That is a worry because the 735 is very representative of the recent state of the Lumia nation, packing as it does pretty standard specs for the current generation of Lumias. So when the final version appears there are going to be a lot of unhappy users shouting at their phones unless the experience has been fixed by then.
So whilst the OS is lovely to look at and use, Microsoft's mobile team has some work to do in ensuring the later builds don't wreck the fluid performance that typifies a Windows Phone device.