If you can get coins and pack of coins in the first week, get them, they are worth as munch as digging and they will save you at least 5 week of upgrading...usefull for saving.
With a philosophical touch...or at least, a clear or a more mysterious story, I would give a 4 stars. I give a 3...something lacks even if it's decently made.
Perhaps you could add some kind of reincarnation option. You know...reset to level 1 and get some extra stats...also, some achievement would look more tempting with time this way...