I’m really enjoying Alumizu. I’m going to list some downsides below, but mostly I’m just nitpicking. I only wish it weren’t so expensive to have aluminum keycaps; this will probably be the only set I own.
Biggest plus: Cool to the touch. Totally different feel from typing on plastic. I can’t overstate how awesome it feels to type on.
Other pluses: Sound is good, comparable to a good thick plastic cap (not as deep as SA though). Looks really nice. Colors not as vibrant as ABS plastic, but the metallic shininess is really neat.
Minuses: Some of the legends need work. The E is a bit anemic in comparison to the surrounding keys. The overall size of the bottom edges is a little smaller than typical Cherry, so the gaps between keys are more visible.
The mounts for these keys are different than standard plastic stems; as with other metal caps, they mount using four square-edged pegs in a cross-shape. This is fine by itself, but the design allows for a small bending of the pegs, meaning that keys can appear to lean to the side once mounted (easy to readjust). The lean is subtle: one picture has an example with the PgUp key (topmost row, second from the right).
Sort of a minus: the keys are VERY firmly attached to the switches. Removing caps is very difficult, especially for a hotswap board, as the switches frequently come out of the plate before the caps come off. On the good side, this does mean that the stabilizers stay in place more securely and the stab mounts don’t slip out (a common problem with some plastic caps).