The most disappointing YC-backed product I tried was Stypi.<p>I teach community college and sometimes I wonder about the thought processes behind some of my students' papers. Paul Graham linked an essay he wrote in Stypi, where you could watch him write it in real time. This was clearly the greatest computer-assisted tool for teaching writing ever, and I immediately incorporated Stypi into one of my writing assignments. I wanted to know how much my students proofread, how they structured essays, and what they struggled with as they wrote. I was so excited about it that I wrote the entire assignment in Stypi and linked my students to the replay in case they were interested.<p>It was a disaster. So many students lost essays in browser crashes or were flat out unable to use the software. I ultimately had to apologize to my class, give everyone an extension, and cut Stypi out of the project.<p>Apparently they were acquired though, so I guess they made someone happy.
You should check HomeJoy. They were a cleaning company (they subcontracted cleaners) that went through YC, had a huge growth and raised about $40M, but went on to fail. Look for them in HN search, it has been discussed a lot.
One simple way to evaluate this would be to look at press attention.<p>I did a quick check using Mattermark data of which YC companies got the most news since 2013 that are not still alive. It yielded (num articles / startup):
67 Homejoy
13 Tipjoy
13 Buttercoin
6 Tutorspree