I'd recommend claiming the upgrade even if you don't intend to install it right away. The free upgrade offer is only valid for the first year it is out. After that it will be a paid upgrade.

It's currently in public beta testing mode now. Overall think of it as kinda combining Windows 7 and 8 together UI wise (though more with the "flat" style 8 has). The start menu makes a return, with the start screen from 8 banished on desktop computers.

Under the hood, a lot of good work is going on to finally let Microsoft distance it's self from some of it's legacy boat anchors, but not in quite the salt the earth way that the Windows 8 UI took. Windows 10 going forward is going to continue to see updates that add new features and evolve the platform. It's possibly the last "release" of Windows in the traditional sense if their strategy works out, moving Windows to more of a software as a service model.

With Windows 10 being a free upgrade for the first year, software developers may be tempted to move to a Windows 10 only support model sooner then has happened in the past. Hard to say if this will happen for sure, especially since Microsoft is opening their application store to legacy Win32 apps, contained in little VM like isolated containers.

Overall as a non Microsoft person, I'm liking the direction it's going.