I really enjoyed my CELTA course.
I can offer some advice: try to get some teaching practice (if you don't already have any) before doing the course.
I would say that if you had to weigh it up (in terms of preparing for the course), the teaching practice is probably slightly more important than the grammar. As long as you have a good foundation of grammar knowledge, you can learn bits and pieces as and when you need to, but with actual teaching it's not so simple.
Good luck, and come back to this thread if you have anything to add on the experience. It can be hard to find "real" information on these courses are threads like this are a valuable source of information for newcomers to the field.