Hi,
I took the CELPIP. I got the highest score 5H in everything, I have been speaking English since kindergarten though so I found it all very easy. Do not follow the advice to exceed the word count for the written part. I was exactly within the word limit for both the written letters. Follow all their instructions carefully. My best advice to you is not to panic they do not ask you anything crazy it's just the basic use of the language. If you are nervous do the practice tests.
The only thing I would not recommend CELPIP for is that you can hear everyone else around you speaking during the Speaking part so that can be either quite intimidating or quite distracting. If you were already nervous it makes for quite a bad combination. For that reason only it might be better take IELTS if you are easily distracted.