This alpha has easily 30-40 hours in it for me.

The funny thing about the starting class is that it only decides which spells you start with. All my mages became dex mages with spears :P
yep, in every Divinity game (except Dragon Commander of course) you were not fixed to the class you chose at the start of the game. This is "feature" became pretty rare in the last couple of years and I like that Larian sticked to this system.
Also I just started my 4th play through and I still discovered a lot of new things, so there is probably a huge replay potential when the game comes finally out!