While attending last year's AMSA conference I signed up with Kaplan to give feedback at an open forum with the authors of medessentials on what incoming MSII student's were looking for in a USMLE review book. Kaplan definitely did the leg work on their latest attempt to compete with First Aid. It seems that the idea behind medessentials is that it is very similar to First Aid in content yet instead of a skeleton outline that you must supplement with notes in the margins they already added "most" of that so you don't have to.
The big question...do you stray from the tried and true First Aid or do you step out on a limb and try something that could be better but just doesn't have the track record. I'm not sure what I'm going to do...I've been using First Aid as a primary review source and if I need additional info I consult medessentials before I crack open BRS or the texts. On a side note...I often find that medessentials presents the information in a way that I can understand and absorb more readily...although that's just my take on it.