Main things:
1) A very rigid structure. Each day a specific lecture is released. You have to do that lecture and can only do that lecture. You can't do any others that have not been released and cannot do any other lectures until you've done all the ones that have been released up to that.
2) Organization of lectures weren't that great. As noted above, they chose which lectures were released on what days. The sequence didn't seem to flow or correlate, but skipped around a lot.
3) Quality was ok. Lectures are made by attendings at the participating institutions, so there's a lot of variation in the quality, content, etc. Some were awesome, some were... not.
4) Lot's of surveys that were required. I guess they wanted a lot of feedback since it's a new thing, but it was quiet excessive.
Most of us turned to the tried and true textbooks. Of course, we don't know exactly what's going to work until we get our scores back. Although, our class ITE performance, which should hopefully somewhat reflect our performance on the Basics since the ITE covers both Advanced and Basic topics, was quite positive, so StartPrep definitely isn't necessary by any means.