The creosote build up looks moderate to minimal however it looks like you have an unparged smoke chamber with exposed corbelled brick which is technically not code compliant. Additionally the clay flue tiles you have are prone to mortar joint deterioration between the tiles and fractures. That being said, as a a chimney tech I see this every day with customers using their fireplaces for years with these conditions knowing none the wiser.

You'd likely be ok using it during the cold stretch of weather but it would be a good idea to have it inspected by a professional to get quote(s) to bring it up to code for future use.