I had one, fallen paladin x his former student (or wife).
The first time I tried it, we had great chemistry between our characters, great build up to the first smut scene, impressive world building, and a smoking hot first scene. Then, it devolved into nothing be smut for the next three scenes. "Let's have sex in this room, now let's have sex in that room. Oh, now we should have sex in another room." I got bored of waiting to get back to plot, and dropped it.
The next time we jumped right into smut, but it really worked for me, great scene. And then my partner dropped me. So three whole posts.
The next got a little further, with a perfectly executed first scene that I absolutely loved. Then it sort of meandered when our characters finally got to meet, before he disappeared. Bummer.
The next was great, wonderful chemistry between our characters, delightful build up for the first smut scene. Then my partner took a long break form the site, and when we started up again, the luster was gone, and the first smut scene fizzled out. But I persisted, and we had some interesting plot elements come up and some interesting character development, and I was into the direction we were heading. In the end, it just didn't work, and we dropped it.
Part of the issue with all of these was that they were pretty generic "dom/sub with corruption" dynamics, and while that scratched an inch, it wasn't the overarching plot I wanted for the rp. I had actually given up on finding a partner for it, and resigned myself to write it solo. I meant to take it off my request thread, but on the day I returned from a lengthy hiatus, I got an approach that perfectly captured exactly what I wanted out of the original idea. That went on to be one of the best stories I've ever written, and I am so proud of it, I've been working on turning it into a novel series.
So, I guess the lesson is don't give up if you absolutely love an idea. But, also, don't treat an idea as so precious that your partner doesn't have room to add in their own ideas.