Anyone know how well cortisone injections work for cystic acne?

Local only. This can be used selectively for acute cysts but are only part of more comprehensive treatment with oral and topical medications. For severe cases Accutane which is used for 15-20 weeks can work well long term. Discuss with dermatologist your history and treatment options.
Cystic acne. I agree. Steroid injected directly into a cystic acne lesion can and does work well but is not always first-line treatment. It is more of an augmentive treatment to whatever baseline treatment a patient is receiving.