In FONV interactions with people were mostly decided by your reputation and because of that I've felt that karma system was unnecessary. It didn't have any large effect on gameplay, and all the things that karma did decide, could easily have been governed by reputation.
Is karma system unnecessary? I would like to know your opinion.

I'm undecided about this. It didn't affect the gameplay much so in that regard it was unnecessary, but if it affects the tone of the endings as it - iirc - did in New Vegas for some of them, I think it could well be retained and even expanded for that purpose. I won't miss it if it is gone in the next game, but it does have some potential for variety of storytelling and playerimpact.

Yes Fallout games should have the karma system because it was so much fun to play until the ending in Fallout 3.

The Karma system svcks imo. At least like how it is now. Maybe the actions you do are not bad to you? And this thing when I'm sniping bandits and fiends and gain karma, what's up with that? I mean, I try to play a bit of a badass character right now, who doesn't get involved unless there's money in it. He's a merc, in short. But why does he turn out to be a good fella, when I kill people that will ambush me because THEY are evil? Evil never fight evil? I try as best as I can in dialogue to not be too nice like "I'll do that for you, I don't ask for any payment", then I kill people that gives me Karma so I'll go around thieving to try stay neutral. I really find it hard, because I don't want to be Stupid Evil and kill people just to get a negative karma. There could be more evil karma options in dialogue maybe, like if you do a job for some civilian, you agreed on 300 caps and when you're done you break contract and want double. That's "evil" in a way. There simply is just too much Good karma options in the game.

The system should get a complete overhaul, don't really want to get rid of it since it's falloutish. But it needs to be much better and have impact, like maybe different dialogue choices. Having low karma and talking to a raider leader, you can impress him. Having high karma and talking to some, I dunno, wuzzy Follower, you can impress him.

Karma System is unnecessary. We have the far superior reputation system. Karma is to complicated for devs to put in the game. No matter what they decide is good or bad, people will get pissed off, and in the end it does effect the game on bit. Its just their for the player to go "hey I am being a good guy look at my Karma!" I long ago stopped carrying what my Karma is in New Vegas. Even with Fallout 3 (even though it was completely useless) I didn't care what it was at.