I've tried both the .98 and 1.21 although I've only had the 1.21 for a few days so I have more time with the .98.
I've used Masters, Blue Diamond, Kamui .98, and Kamui 1.21. I'm a casual player who plays mostly at home for about 30 minutes a day.
My overall impression is that any chalk is fine but the Kamui chalk definitely stays on longer and you don't have to chalk nearly as much. Because of this, I think the price isn't that bad. It seems crazy, but if I use the Kamui a fifth as much the price is more in line with blue diamond.
For me, the best thing about the kamui chalk is that it is not as messy. I started off with the Masters and when you apply it half of it goes onto you tip and half goes on the table, floor, and your hands.
I then moved up to the blue diamond and thought it was great. it was half as messy as the masters and I thought it was great.
Then I tried the .98 and almost 100% of the chalk stayed on the tip, no chalky dust flying everywhere. The downside is that it is very sticky and I think it gets on the cueball more.
I then tried 1.21 and initially I think I really like it. I feel like 95% of the chalk stays on the tip and it is a little less creamy. It feels more like a traditional chalk. It is between blue diamond and .98 but much closer to the .98. I think you have to apply it a bit more than 1.21 but certainly not every shot.
So in my opinion, I think I like the 1.21 the best. I think the kamui chalk might have a slight performance gain but more importantly for me, it just keeps my carpet, table, hands, and cue clean because it doesn't flake off and get chalk dust everywhere.
It is expensive but I use much less of it so it might cost 7 times as much as blue diamond but I think it might last 7 times as long so the cost increase is a wash.