Most course books/packages have a shelf life of, say, 10 years at most, then need replacing. It's one of the big financial and methodological decisions an MFL department has to make and should be taken carefully, involving all members of department. I know from my own experience what it's like to be lumbered with an inappropriate course which forces you to write new materials and dip into other sources.
A good course book, kept individually by students, is a great asset which should save the teacher a huge amount of time and provide a very useful resource and comfort blanket for pupils.
So what criteria should be used when weighing up the options? Here are 20 for your consideration:
Is the language material in the courses rigorously selected and graded for difficulty?
Does the course have the right balance of grammatical, functional and situational material for your needs?
Does the course have a sensible grammatical progression with built-in revision? (Check for spiral curriculum structure year on year.)
Is there sufficient cultural content?
Is the difficulty level right for your school or classes/ability sets?
Is there a sufficient range of materials for all the skills, possibly including good online content?
Is the content relatively timeless? i.e. will it still look up to date in ten years?
Does the teacher's book or support resources have creative and practical ideas for lessons?
Does the course prepare students effectively for examinations?
Does it have the right amount of target language?
Is the pupil book clear, with a grammar section and verb tables?
Is the reading and listening material interesting for students?
Could your department all work with it?
Do the exercises in the book or worksheets have enough examples to allow for rigorous practice and are they pitched at the right level?
Are tasks and pages clearly numbered for easy reference?
Does the course have a built-in assessment pack?
Are listening resources clear and at the right speed?
Do nearly all exercises look usable?
Is it clear from the course what underlying methodology it is based on?
Is the online content easy to access in school and at home. Is it stimulating and ungimmicky?