I always thought that a 65% would be the perfect size for me until I got my NightFox.
The layout is practical and I think it works well for most because an arrow cluster is useful.
But coming from an HHKB, where I've daily'd it for over a year before my fox, I just find 60% more useful. The main reason is that I never look at my keyboard, so knowing that the backspace and enter is at the very edge of the keyboard helps me get my hand oriented faster. For this reason, the FC660C was actually better for me than a 65%, but without a split backspace, it wasn't perfect. I could have hasu'd it but I didn't want to put more money into it.
I've long considered building a 60% with a modified arrow cluster at the bottom right, but I think I'll still prefer a HHKB. The default arrow cluster on the function layer is fine for me, even when coding.
I think the build quality of the ALT is fine, my coworker right by me daily's one. I personally don't like LEDs and floating keys (not sure why I got my NightFox), and I don't particularly like hot-swap because of durability, but the ALT has a lot of things that people want.