I've owned cars with sunroofs, moonroofs, t-tops, removable hardtop, bikini top, soft top, and convertibles. For a Mustang I prefer the convertible, but the fastback style also calls out to me.
Out of the vehicles with modified roofs, the fabric style convertible that I have on my 2013 Mustang is the best of the bunch. It doesn't leak(yet?) although I don't expect it to. It does squeak/rattle a tiny bit over rough roads when the top is up.
The a/c can be used on days like today when I was motoring with the top down. As has been said, evening cruises are the bomb. For daytime running, keep sunscreen and a hat as well as a few towels in the trunk. (towels to cover the seats when they are in the sun and you are parked). I haven't tried the new Mustang seats with cooling so can't say anything about them.
I really like the idea of roof/ no roof at your demand and my car is just a Summer only vehicle. I thought it tracked well at TMP, but I'm sure the solid roof version can be tweaked way better.
I'm very happy with mine.