"have at it" is a correct and usable phrase in written English. It typically means to go ahead and try something, or to do something enthusiastically. For example: "I have a project that needs to be finished by tomorrow. Have at it!".
Readers, have at it!
Have at it.
"Have at it," he said.
Have at it, Caleb Hanie.
PERRY: Great, have at it.
All other subjects — have at it!
"We have practice at it," he said.
The companies themselves have hinted at it.
Carol and Joseph McLaughlin have been at it much longer.
I shouldn't have swung at it.
"The players have stuck at it".
Awesome tool! I started using it one year ago and I never had to look for another app
Ha Thuy Vy
MA of Applied Linguistic, Maquarie University, Australia