Dense, hard Decio impressed me as a likely keeper apple when I tried it for the first time last November. So I held one back to eat near the end of the year.
My sample's sweet aroma is leavened with a yeasty note that is probably related to its crown of russet.
Otherwise it similar to my November sample. I'll add that Decio's stem is thick and its calyx is wide open.
The flesh is a little grainy now, and in more ways than one. The apple's texture has started to go granular, and the nutty, toasted-grain flavors I noted in November are more prominent.
The apple is still dense and on the dry side, but has softened slightly. The progression seems similar to that of other hard, dense keepers, such as Winter Banana and Arkansas Black; see for example this report.
Decio's other flavors have melded a bit, but I think I can find some pear and vanilla in the mix, which is pleasant and well balanced. I wouldn't say it was especially spicy. There is a tiny bitter note from the peel, but it is fleeting.
Even in late December, Decio is good to eat, and it retains its fundamental character as it ages. Nonetheless, if I had a stash of Decios I would plan on finishing them off before the Solstice.