Do "posture chairs"really help your posture when at the computer?

Yes they can: Yes, you need to keep your back straight and let the natural gravitational alignment work for you. Try to keep your shoulders down and keep your work as close to eye level as possible. Every now and then, check your posture and ensure you don't slump and keep as much of your work at arm's reach and try not to lean to get your work. If need be, get up and bring your work close to you.

Yes: An ergonomic chair is very helpful for patients who experience low back pain with sitting by maintening the best posture.

Yes: Proper lumbar support will shift your pelvis forward which will correct your sitting posture. Make sure that your office chair has adequate lumbar support.