I did this year's AoC in Ada. (Well, part of it, I didn't do the whole challenge.) I'd never written any serious Ada code before, and this was a great way to get a better feel for it. Overall a positive experience! Ada can be a bit verbose at times, but it wasn't nearly as bad as I had expected it to be... Once you learn not to fight against the grain of the language, it's just a matter of decomposing the problems in an Ada-ish way.

Suprisingly I found it very constructive to work on "two puzzles at once" -- how to use the new language, and solving the AoC itself.