I've played through RE5 in co-op 6 times with 6 different people, and for a few of them I bought them a copy to play with them (because screw split-screen), and I love every second, every time. RE5 has been the best co-op experience I've ever had with a game.
RE6, however is one of the worst co-op experiences I've ever had. I played through it once at launch, and my co-op partner had to drag me through, because it was such a bore and devoid of any satisfy gameplay mechanics, and riddled with glitches, cheap deaths, lame bosses, and terrible and goofy shooting mechanics.
One of my newer friends had never played RE5 or 6 before, so last summer, I bought us both PS4 copies of 5 and 6 to binge through over the course of a week, and we did. As expected, we loved every minute of RE5, and RE6 had us wanting to quit multiple times. It's so hard playing 6 after 5. It's like going from a polished, focused adventure to a shoddy mess of a slog. I was hoping it would have been better than I remembered, but it was worse.
Anyway, my girlfriend has never played any RE game except for 7, which we played through together at launch not long after we started dating. We're about to finish RE4, which she's loving so much, and I just bought her a copy of RE5 to play together right after. I actually thought again of getting another copy of 6 to play after 5, but after recalling the vehicle portions, the drawn-out and lame boss fights, and cheap deaths, I removed it from my cart and am never looking back.
Also, did they ever patch out the ability to dive on your back and then shuffle up stairways? That was one of the goofiest things I've seen in a game outside of Sonic the Hedgehog 2006. In fact, as far as I'm concerned, RE6 is practically Resident Evil's Sonic 2006, as they both ruthlessly broke my heart and I hate them so much.