Bishop, Knight & King versus King.

Just for the record, mate is forcible with this material configuration, so it's definitely possible to accomplish.

No, no, here like in many other basic mates (think R+K vs. K) the idea is rather to avoid checks and instead use the Zugzwang to corner the king.

I only recently learned this mate myself (I found the explanation in Yusupov's book Boost Your Chess 1 quite good). I don't think it's very important skill in itself (I've never had the position in an actual game) but maybe practicing it can teach some other very usefull skills like coordinating the actions of different pieces and planning. However, for someone rated above 2000 (in otb) it would be somewhat embarassing to faill to deliver the mate the one time in your life the position actually comes up in a game.

It's complex usually takes around 25-30 moves to do it correctly with no mistakes and it's not elementary takes some practice and work to master it.

I've practised against the computer, and managed to give mate. It's not very difficult when you know how to outmanoeuver the king, and trap it slowly. It can be done in 30-35 moves.

But honestly speaking, I never got this endgame on the board.

Don't you mean it's overrated? You are right, though; other endgames are much harder. Still, I do believe K+B+N v. K is the most difficult of all endgames involving a bare King for one side.

In chess.com computer workout there's an exercise on this theme with the king starting on the "wrong" corner rated <1200. Now that's what I call underrated!

 ever try the K and 3 knights verses bare king

Okay, you may have a point, there. 

 took my opponent 45 moves to do, and I made a mistake at around move 30 and he almost lost that one due to 50 move rule

Life is too short to worry about such infinitesimal occurrences.

Not once in thirty-eight years of play have I experienced this endgame.

I don't know how to complete it and I don't care either, it's just not worth the effort.

The statements by some beginners, that they find the technique easy, is to be taken with a pinch of salt.

According to the db on my computer, I played this game here on Chess.com. I am not able, however, to locate this game in my archive on the site, nor am I able to locate my opponent.

were those underpromotions necasary?!?

Yes, so that I would have an answer for forums like this one.

On ICC the rating for the mate is 2200. I definitely say that's reasonable. It is just the fact that once you learn it, you know it for life. I could teach you guys it in probably about 10 minutes if you were interested.

One endgame I really want to learn, though, is K+Q v. K+R. I have never reached that endgame yet, but I would be very upset if I drew that endgame just because I didn't know what to do. I still can't get the hang of it. It's so difficult against the best defense!

K+Q vs. K+R is ridiculously hard. If you make even 1 mistake, you instantly will pretty much be guaranteed to draw. I can't do that endgame either, but there are 21 positions you need to know in order to master that endgame. The most important is the philidor I believe.