Hello,
First i want to say, that a beta is always good for a finsih game without bugs.
If a Beta is open, many players test the game and help to find bugs.. And If there are a
lot of bugs in the beta.. The Producer of the Game can find them and can kill them 
I play not very often, but I am from Germany (thats far away too (from USA) ) an my ping is beetween 100-300
I think it depends on your download/upload and on the host of the game too, for example if the pc of the host, is lagging, you won't get a
low ping .)) ( So i think)
To answer your question: Should i buy the game?
If you are sitting in a room, with 6-10 players and all play Demigod, you will have a lot of fun.
For example, you kill somebody and then he will cry :" Cheater, that was a lag.....
"
I think the game will make a lot of fun in this way, but if you want to play singelplayer, this game won't make so much fun like killing friends 
I´like dota in WC3, too, and it makes a lot of fun, and so i think, that Demigod will be a good game.