I would say try the demo/trial out and see if you like the game play style first of all.
Its not hard to jump into, the previous games are not required to understand the story and there is a lot of lore to find which can flush out the story a bit more.
You might not be able to catch up to some of the GC people right away, but you can play from start to finish in single player (even though you are still online).
I am enjoying the game so far, and I can only imagine Blizzard will continue to improve upon it as time goes by.
I also have 1 free game demo code left if anyone wants.
And Preacher : It takes a LONG time for Blizzard games to have a big price drop. Considering 3.5 million copys sold the first day.