Can't believe I didn't comment in here since I do use chapter summaries pretty religiously at least with UMS. I haven't integrated it to my other stories yet and I'm not sure if I will. I think it works really well with UMS just cuz its such a complicated and twisted plot that like Tri does it even helps me as a writer to remind me what happene in each chapter when I need to refer back to something. Usually I just use a sentence or two at most just vaguely summarizing what happens in the chapter. So for example, my summary for lets say Chapter 26 says : "Alyssa fills Izzy on what's been going on between her and Nick." Very simple and to the point, but you still want to read because you have no idea how Izzy is going to react. I try to write just enough to pique the readers interest.
As far as putting in individual chapter warnings, like for sex scenes, I don't bother. My story is Rated R and it does say there may be graphc sexual content on the front page. Sometimes you can tell by the summary that there might be a sex scene like in Ch 22 the summary is "Nick and Alyssa finally give in to their feelings for each other." and Ch 23 says "Nothing is as simple the next morning." So, yea obviously if you have a brain you can probably connect the dots, but as a reader I don't like knowing there's sexual content in a chapter before I begin reading cuz it ruins the surprise for me. Plus, you get those readers who just want the characters to finally "do it" so they are prolly going to skip right to the sex parts.
As far as actual story summaries go, I think two to three sentences is more than sufficient. And I also hate when writers go OMG I suck at summaries! I think thats pretty unprofessional and its going to make me doubt your abilities as a writer. If you don't even have the confidence in yourself to write a simple summary then how am I as a reader, going to appreciate your work?
Lastly about the NC-17/visual debate... I've read some extremely well written NC-17 stories that were not at ALL smutty. I do tend to think a lot of writers over rate their work cuz I mean I see NC-17 rated stories that only have mild lanuguage in them or like ONE sexually explicit scene. I do think there is a major difference in the two, but thats why I try to shy away from the NC-17 rating. I'd rather keep my stuff at R so more readers will be inhibited to read it.