Raze_Larian: That depends on whether or not you wish to participate in the alpha/beta. Are you interested in the development process? Do you want the option to provide feedback on systems and features, possibly influencing the game design? How inclined would you be to try out different characters and/or different party builds? Do you tend to play RPGs only once, or play/restart multiple times trying out different approaches, looking for effective / interesting / challenging ways to play a game?

Only the first act is included in Early Access. There will be some content added and refined, but no new areas will be included. There will be features added (new skills, and only the basics of crafting are in now, for example), and balancing changes made to various systems, etc. Given the reactivity of different origin stories and multiple ways to solve main plot quests, do you think an update or two would be enough difference that you could play through again?

Saves made during the alpha/beta will not be compatible with the released version of the game. Any updates with significant changes (such as the one in the process of being prepared to be tested) will break save compatibility, but you can always delay updating or revert to an earlier version to continue an existing playthrough.
After release, all updates will maintain save compatibility, but too many things are being changed and added during the alpha to do so.