Hehe, and the sentence instantly feels different when you change 'so far' into 'until now'. His job has been easy so far. (His job has been easy and we have no indication that it's going to be any tougher.) His job has been easy until now. (His job has been easy but it's getting tougher from now on.)

Until means " up to the time when " , " up to " Example : He had to wait until the rain stops . He can't leave until he finishes his work . So far means "until now" ,"up to the present time", Example His job has been easy so far. / So far his performance has been good . So far so good ( expression ) which means "up to now everything has gone well "