Yes, I did the CELTA a few years ago and can confirm it's hard work! I know a lot of people who have done it and more often than not it is described as the most intensive academic 4 weeks of their life!

There is so much to take in that all prior knowledge helps. You can take it without experience of course, but my belief is that you'll find it much, much more difficult than if you have good grammar knowledge and experience. For one thing, you won't need to spend time learning basic stuff like the different verb forms and tenses when they arise and can concentrate more on the teaching methods and activities talked about in class.

When the teacher talks about different ideas in class, you'll also be able to relate much more to what you have experienced already and thus be able to make more critical and informed judgements about it. I feel that I got a lot more out of the course because of my previous experience than had I gone in and taken it blind.