This review assumes you have watched the Onegai Teacher TV series first. If you haven't then watch the TV series first because you will probably have problems following the main points.
There is no new content at all. Aside from the first minute (which takes scenes from a later episode), Reminiscence is nothing more than putting all the episodes back to back and then cutting it down to an hour. It appears to be an edit of the main series that was done fast and cheaply.
The scene changes are abrupt, and they do not flow particularly well. A lot of
the character development and virtually all the comedy aspects are taken out, making the tone more serious, especially in the first half. If this had been expanded another half hour to be a standard recap movie, a lot of the problems probably would have gone away.
Reminiscence wasn't terrible, but isn't worth the time. It ultimately felt like a quick way to cash in on the series. It ignored a lot of the story and character development, and there were several instances where a scene is cut off immediately in order to cram everything into less than an hour.