As I've mentioned before, in nearly every country but the United States, Smarties are candy-coated chocolates made by Nestle. Until I was in my 20s, I had never tasted these non-sugar disk Smarties, so this review is probably biased.
Naturally, I can't fight the temptation to compare them to M&Ms. Right away, the different colors of the Smarties appealed to me (pink and purple as standard colors!), and they were also slightly bigger than plain M&Ms.
There wasn't much of a scent (the cardboard smelled stronger than the candy), and the taste immediately reminded me of Halloween chocolate - a blend of candy and chocolate flavors stewing together in a vat. The candy coating seemed to add a sugary flavor, and because it is pretty thick, these were very crunchy.
I was reminded of Cadbury Mini Eggs (the chocolate eggs coated with a spotted candy shell to look like bird eggs) as well, but Smarties are much smaller than those. As a result, the chocolate seemed dominated by the shell. Although I don't think I'd turn these down if they were offered to me, I think I'd prefer plain M&Ms...but that's only if plain M&Ms and Smarties were my only choices.
Check out this head-to-head comparison at Candy Blog.