myshell - this MH serum might be a great overall product but over time, Retina A is always going to be more effective, just due to it's strength. But it can take at least 6 months or more to start to really see those results (of turning back the clock, so to speak.) However, when started on the younger side, retina A will make a far bigger difference in the prevention of aging which, in essence, equates to the delay of actual aging of the skin. If I recall correctly, you are on that "younger side" still, yes?
With the additional ingredients in this Mad Hippie Serum, your skin might respond more obviously in the short term yet that would probably be more of a result of the other ingredients rather than the retinol itself. Hope that makes some sense?!?
As far as where you can apply retina A, yes, you can use it both on your neck and up under your eyes. I did both (as I know many others also did) when I used it and had no problems. If you do notice any sensitivity, then apply a cream first and the RA on top. Was just reading an article by Dr Brandt from last year who said that was how he always used RA and the cream actually helped in the delivery of the RA.
Another choice might be to use RA at least 3 times a week and the MH serum the other 4 nights (or visa versa.) That way you would still be getting the long term benefits of RA along with the short term benefits of the MH serum.
The frustrating thing about RA is the long term commitment required but I doubt you'll ever regret making that commitment as the benefits really do pay off from doing so. Retinols are absolutely fine for those that can't tolerate RA but they will never out-perform RA as they just do not have the strength of prescription strength Retina A.