I consider $2 impulse-buy money, so I got the DLC. It starts off the similarly, but about half-way through ustwo seems to hit their stride. The last level has some flat-out brilliant Fez effects, and a few of the levels really play proper homage to the Escher design. My math says you get 2/3rds the game at about 50% more quality (integrated over the whole run) at half the price, so that's like twice the value. Compound levels are used more, so the game pleasantly drags on a bit. Just don't expect to be confused or stuck on any puzzles.
I won't lie: it's still short, and it's still easy. But it's also two bucks, which is slightly more than a candy bar and a bit less than a load of laundry. And I tend to enjoy the game art for its own sake.
So I think the game is still mostly an art project and it stands on its own in that respect. While FS doesn't expound on the plot much, I don't think anyone will mind too terribly. But there is some thread to follow, and for a game without much of a plot there are certainly twists.
But it's easy to forget just how unusual those games are. There's a dearth of well made Escher games. I can only think of half a dozen off the top of my head, and perhaps none that are simple enough to be on the iPhone (4S in my case). Now, we can argue about what counts as well-made, as there are a few that were acclaimed that I didn't really like (I'm looking at you, The Bridge). But MV really is a fairly unique take on the idea, and about half-way through Forgotten Shores really begins to wake up into its own.