I'm going against the wave here, expecting downvotes already but I think I may have a good point.

I have over 100 hours on the original and it is a great game. I started playing all kinds of roguelikes because I loved the genre so much. Rogue Legacy, Risk of Rain, Crypt of the Necrodancer, Faster than Light, Don't Starve and a few others. The thing is, when I went back to play The Binding of Isaac (the first one) I found that I was not having any fun because if I got unlucky on the first rooms and get a couple of bad items that was it, the run was over. It was still possible to beat the game but it would take so much time and I wouldn't have fun with the game.

A good example that comes to mind is the speedrun at the AGDQ (Awesome Games Done Quick) event, which was really big this year as people would speedrun games and gather money for charity. The 'The Binding of Isaac' run was a race between two players, some games have runs like that and it's really cool. It happened that one of the players got lucky and beat the game in 26 minutes and the other got unlucky and died at 40 minute mark on the last boss. You should know that both were known streamers and really good players, so yeah, skill is nearly as much a factor as RNG. You could argue that the game isn't suppose to be possible every run which I think is a flawed design. The skill is capped until you learn about every mob and how to play against it and that's that.

Back to my story, months later after stopped playing the original, Rebirth was out. I decided to give the game a shot because why now? I bought the original in a huge sale and the least I could do was to support the devs. From the start, I disliked the visuals (even with the filter off) and the new soundtrack (even the 'voices'). Played a little more and beaten the games about a dozen times and didn't notice anything worth the money different from the original one. Yeah, it runs better now and has controller support, which is really cool, but I used controller on the original one aswell.

So that is my opinion, it may be biased as I lost the love for the game but consider a few of those points. The remake was totally unnecessary IMO and they are already planning an expansion (I don't like using the term 'cashgrab' here because I don't think it's exactly that because of the story of the devs and being Indie and stuff, but that sure feels like it). If you have some extra cash laying around, go for it, but I wouldn't buy them expecting big return if you already have the original one. You could say Rebirth is a slightly better game though and I would be fine with that.