Not half bad
Definition 1 of 1
Pro Tip 1/3Positive Surprise
Indicates something is better than expected, often used after initial skepticism. 
The band was not half bad; actually, I'd see them again.
Pro Tip 2/3Understatement
Sometimes used to modestly understate one's positive opinion about something. 
You said your cooking wasn't great, but it's not half bad!
Pro Tip 3/3Informal Praise
Used in casual conversation, more informal than direct praise. 
Your idea for the project is not half bad—let's discuss it more.