I just bought the Sega Genesis Collection on PS4/Xbox. I had one as a kid and loved it but put most of my money and time into the SNES. It's nice going back and being able to play a lot of the games I didn't get and finally beat them.
Looking on it with modern eyes, Genesis games just feel like NES games with better graphics and sound. They all seem pretty short and a lot are really challenging. It makes me appreciate the Genesis a lot more and how drastically things changed when SNES came around. It became more about the experience, storyline and length rather than a quick game experience.
One thing I love though is they have a rewind and fast forward feature. Get hit, just rewind with the L trigger. Yeah, it's cheap but if you just want to play and beat the game or learn the patterns, it's great. The Mega Man Legacy Collection also has the same feature.