some are interesting - that's about the extent of it
for code, the challenge is often something like "least lines" or "least code"
which only has meaning if you are writing ROM-able code - lol
well - the boot sector challenges on OsDev.org are a little bit interesting and educational
the hackaday site i posted earlier has hardware hacks, as well
some of them are very interesting - and can lead to real-world product innovation
sometimes, they offer a little fuel for thought, if nothing else