Screencastify on and off since it first came out. Recently, I have begun using it as my primary screencaster for several reasons.
Unlike SnagIt, which emphasizes Google Drive syncing, Screencastify took the approach of offline access to your screen recording (Chrome Browser) with the ability to later download or upload to YouTube / Google Drive. I personally like to review / edit my screencasts prior to publishing them to Drive or YouTube.
Another big feature is when I share / upload to YouTube I can chose a specific channel and if it should be private, listed, or public. I have several channels so this was a highly desired feature. Previously, I had this awkward dance of syncing to Google Drive, downloading to my computer, going to appropriate YouTube channel, and then uploading my screencast.
A couple of extra features I like about Screencastify is recording webcam directly, inserting embedded webcam on screencast, and most recently cropping a video. The latter allows you to crop out edges of a video similar to a photo.