TOPIC: "So far" and "Up to now"

Could you possibly tell me whether there is any difference "so far" and "up to now"? Could you please tell me if I interchange the two terms freely?

Thanks

Both "so far" and "up to now" can be used to mean "until now", but I cannot say for certain that they can ALWAYS be used indterchangeably, for I'm sure we could find some exceptions. Probably in most cases, the two expressions can be used interchangeably.
HOWEVER, it is important for you to know that "so far" is used much more often than the other expression.

Also please not that "so far" also means:
1)to this extent, the degree that:"My boss even went SO FAR AS TO suggest that we might take back ownership from Marketing."
This may be written as:"in so far as ", or "insofar as", or "so far as" and is used as a conjunction (formal usuage):
"The story is based insofar as possible on notes made by Scott himself."

By the way, have you heard the expression (and know its meaning), "So far, so good."?

Thanks for your quick reply.
I think it's very useful phrase, "So far, so good". That means I'm fine up to now. I think it can be used such situation.
If you meet your friends and your colleagues, maybe you are asked how about you .
You can say, "So far, so good".

Yes ,you're right about "So far, so good" meaning everything is okay up to now. But please note that we would normally use it for something that is IN PRGRESS, i.e.,
A:"Hey, West, how's that big project going?/ How's your job going?/ How's your day going?"
B:"So far, so good."
It is POSSIBLE in response to the question, "How are you?", but not so usual.