'that's great' is correct and usable in written English. You can use it to express enthusiasm or approval. For example: "We just broke the sales record - that's great!".
"Yeah, that's great coaching.
Wow, that's great.
OK Glass, that's great.
And hey, that's great.
Obsession, that's great".
That's great business.
Girardi said: "That's great.
Thanks to Ludwig my first paper got accepted! The editor wrote me that my manuscript was well-written
Listya Utami K.
PhD Student in Biology, Bandung Institute of Technology, Indonesia