The museum is small. Allow half an hour for an overview, an hour for a thorough visit, or 90 minutes for an in-depth study. Note that certain parts of the museum are open only as part of a guided tour, so check the times of these before visiting if it's important to you to see it all.
The museum traces the medieval history of the Order and then skips ahead to more recent history with a small exhibition describing the Order's work in the modern world. I was more interested in the first, more historical, part. There are a few artworks of very high quality and quite a number of other historical artefacts that are interesting. There is an interesting, though architecturally undistinguished, chapel, underneath which is the crypt, which is of greater interest.
The staff are all very helpful and friendly.
Not a must-see attraction, unless you have a special interest in the historical areas covered, but well worth popping in should you be in the area, or if you're a Londoner who has exhausted all the mainstream attractions.
If you own or manage The Museum of the Order of St John, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.