I got hooked on Mephistos and ordered the Helens based on a friend's recommendation due to my bad knees.  I almost immediately ordered the Hale sandal in a very fun, bright silver color.  Loved them, as well. I then got the Agacia for a pair that was a little dressier and not quite as chunky looking.  I'm pretty disappointed in how these feel on my feet, compared to my expectations.  There is practically zero arch support and the cushioned footbed is not nearly as nice, compared to the Helen/Hale models.  They are cute and, if I had tried these on before the other models, I probably would have been sold.  I got these at a discounted price and will keep them, but I would not pay full price for them.  Compared to other cute flats/sandals, these are definitely the best.  But, if you're hooked on the Helen/Hale feel, I would purchase with free shipping/returns and see how it goes.

As far as fit goes:
I am a 7.5 with a slightly wider foot, if not average.  The Agacia in a 38 fit me very well in length.  The way the foot bed is shaped and the shape of the strap mean that sometimes the inside part of my foot twists out of the sandal and almost hangs out when I walk.  Kind of annoying.  Maybe if you have a "taller" foot or higher arch then you could keep it in place.  I have very low arches.  For reference, I have 38s in the Helen and Hale (I thought the Helen ran a tiny bit bigger than the Hale) and love them to death.

For those who haven't tried any of these sandals - I wish I had ordered all of the types with free shipping/returns and tried them on side by side.  We don't have a store here, and mail order is the easiest option...Amazon, as always, has been unbelievable with returns/shipping/support etc.