Is using a wireless iPad as a second monitor any good?

Yes, but it’s not perfect. Apple’s Sidecar works wirelessly with a recent iPad and MacBook Air, but latency is noticeable even on strong Wi-Fi—fine for static content like email or reference docs, but frustrating for dragging files or video. Third-party apps like Duet Display or Luna Display are similar over Wi-Fi; better over USB, but that defeats wireless. Battery drains faster too. For a sidekick monitor for chats and notes, go for it; for real responsiveness like design work, plug in a cable.