I have a software startup and was in SS in 2017 with an assigned mentor and a second time in 2018 but without an assigned mentor. The 2017 SS experience was a lot better because of two things: 1. My cohort of startups was quite involved in meetings and forum discussions 2. our mentor has an agenda for weekly group meetings and gave every group a chance to talk about their progress. It comes down to these two things -- cohort and mentor -- and YC picks them for you, so YMMV.