Looks interesting. I've rented cars through Turo. Compared to that, this looks easier for someone hiring the car because the car is available for instant pick up but less convenient for the person hiring out the car because they effectively surrender their car for half the month. I guess if you had 2 cars and only needed 1 on a daily basis, this could pay for your motoring.
Turo works well for hiring with a few days notice as there is a bit of to and fro with the owner. The keyless entry with Karshare is great for both parties but I wonder what the damage inspection at pick up and drop off is like - could be open to abuse / arguments.