A free product like this will never be perfect, however it does what you want it to do, in terms of being able to simply edit together a sequence of visuals with some audio and text. Simplicity is the key, Movie Maker and for MAC users iMovie cannot be compared to the powerhouse software of the video production industry such as Adobe Premiere or Final Cut Pro. If the tool is simple then it will not take away from what is being taught/learned, however if countless lesson must be spent on teaching a tool then undoubtedly the tool has become the primary learning outcome.
The current generation of students pick up basic computer programs extremely quickly. This quick 5 minute video tutorial on the basics of Movie Maker as well as this hand out should be more than enough to get your students to level of being able to start editing their 'movie' together.