I have a Flash 65 and like it a lot. However, The sleeve holding the frame sheet is a bit fragile. Mine tore and i needed to make a pretty major repair overlaying the stress points with much stronger material. I have seen a number of reports of similar damage, so maybe that is a weakness of the original flash models. Compare one with the new version to see if stronger material and construction is used in the replacement models.h
I do like the pack though. It is for me very comfortable and carries a 30 lb load well. I find the shoulder straps and hip belt to be really good, and wonder how they have been able to "improve" them in the new model.
Re size: I was brought up and trained in the (Brit) army style of backpacking - absoluteley nothing on the outside of the pack, and that indoctrination sticks, so I go for the larger size so that ALL of my shelter and sleep system can go inside.
I also have an REI Flash 18 as a day and summit . Outstanding IMO. I use it as my EDC going to work etc. For weekend grocery shopping, I use the Flash 65.