I swim 1-1.5 km three times a week for fitness in addition to waterfit and was specifically looking for something that would help me work differently, adding variety and challenge to my swims and taking some of the workload off my shoulders.
With that in mind, I wasn't disappointed. These are DEFINITELY different. They are ultra-light - as in floaty and hard to keep your feet under water; while normally you're working your core to keep your legs HIGH, with these, you're working your core to keep your legs DOWN. In doing so, it seems like you need to use a whole different set of muscles.
I've only had the chance to use them for one swim so far and unlike other fins I've used which were really intuitive, it feels like there must be a learning curve to these, so I think more practice will help. I was moving way slower with these fins than usual (others I normally pass were passing me today!) but felt like I was working harder, and in the end, that's not a bad thing in terms of fitness.
The added bonus was for sets on my back, finally my legs weren't dragging for a change - yay! Maybe these will help me establish solid body memory.
I normally wear a women's 9/9.5 and the size 40/41 suggested by SwimOutlet felt rather large, but not uncomfortably so; let's hope they don't stretch, though! No need for a sock at all; they're soft on the foot and comfortable right out of the box with no rub spots anywhere. No strain whatsoever, even with arthritic knees - and no foot/leg cramping at all - a surprise for a new fin.
One last thought; these are tough to keep up a six beat kick for me; they worked best for me at a four beat kick - an adjustment in itself.
So, the jury is really still out on the Arena Tech Fins; time and practice will tell if they actually make me a better swimmer or just add much needed variety to my workouts.