Is French Defense a good opening to stick with ?

Caruana, MVL, Duda, Nakamura, Grischuk, Rapport... yes, played at the highest level.

But as always on amateur level: focus most on the opening principles, not on the opening.

Yes, at the grandmaster level it still sees some play since it's not as drawish as e4 e5, but not as risky as the Sicilian. The Caro-Kann seems more popular at the moment, but openings come and go out of fashion all the time and there are plenty of excellent existing grandmaster games that you can study. There are also a good number of specialist grandmasters who play it all the time.

At lower levels, it's still a great choice since there aren't that many great sidelines that you have to worry about. There are even some ways that you can reduce the amount of theory that you have to learn (ex: playing the Rubinstein dxe4 against both Nc3 and Nd2).

It definitely is. I have a French Study where it goes over the main variations you will face and it is very good for learning positional chess. The only thing my studies don't cover is 2.Nf3 because I'd rather play the Sicilian with 2...c5 instead of potentially facing the Exchange Variation with 2...d5 3.exd5 exd5. 

I wouldn't worry too much about 3.Nc3 or 3.Nd2 too much as from experience, hardly anyone plays it at your level going for either the Advanced, Exchange or 2.Nf3

I've played it for over 25 years and have beaten many with it rated over 2200 Over the Board.

I'm not going to claim I can beat 2600s with it, but it has been played at the top level and there have been many wins by Black at the 2600 level.  Just because you don't see it often in the top 10 doesn't invalidate the defense.

At top level it is not good or reliable anymore but up to GM it is still good and reliable.

Yes

Do you like playing it?

Are you having success with it?

If the answer to the above questions is 'yes', then you should stick with it.

Yes,of course

It is easy to beat. So if your opponent is a french mime...you are expected to win as white.

That is a joke when you see French defense OTB.