(Apologies to any Brits reading. You, of course, already know this full well, having grown up on Smarties and don't need anyone to tell you, least of all me.)
It's not really possible here to explain just what makes Smarties so different (some might even go so far as to say "better") than M&Ms. I recommend you just get hold of a packet (or box or tube) of Smarties instantly without further ado. And sample these Fine British Smarties for yourself (CAUTION: accept no imitations--make sure they're really Smarties and really British) and I think you'll see what I mean. (The best place to find them in the States are either in British shops or in Blighty herself--smuggling them back in your suitcase by the bunch-fulls apparently, as some people I know do, so I'm told, I gather, as far as I can tell)
Here's a very sensible article all about this very thing (it's what I've been telling my friends for years to no avail)--all about how Fine British Chocolate really is (from the New York Times no less, so it must be true): click here
Before you go, though, here's some other Fine Lovely British Chocolates.
This concludes Part 1 in my "Chocolate Series." Stay tuned for Part 2 where I will dedicate an entire blog to Maltesers (not pictured above and my own personal favorite) because they merit it, so much mystery surrounds them, and such terrible confusion exists in the States about exactly what they are. All you need to hold on to right now, and most important of all, is this: they are NOT, under any circumstances come what may EVER, Malted Milk Balls (anymore than Smarties are M&Ms). It would be wise not to forget this.