Ok, let me start off right here before I get farther into this and get a lot of bantering on here about how special chalks are cheating or "I never use special chalk" or "just play the game without it"... I know all the opinions and feels about using the special chalks.
I have never personally have never used special chalk. I would not use it online either. With that aside...
I usually play on hard with no lines but rarely do I win a match. With lines on and hard difficulty, that game is still pretty challenging if you happen to miss a shot. The NPC usually can clear the table without missing. I've been playing through career mode and am now stuck on Professional league. Some of the matches without lines are really challenging and I usually run out of credits and have to do some trick shots or something to get up to enough to attempt it again. I've read online that I should consider buying chalk to help get through career mode. Does this stuff really help? Does it work on the matches without lines?
