The point of these is brevity, it is the soul of wit after all.
Terraria at first seems like a cheap 2d cash in of the minecraft hype, but Re-Logic found a way to make an incredibly deep game out of very little complexity.
The 2d Minecraft comparison holds up to a point, in terms of world building, but there is quite a bit more to Terraria.
They've added some MetroidVania style combat as well as their own take on Redstone-like mechanics. The game has something for everyone, and it all gels together nicely.
No doubt you'll find something you like doing in these simple, garishly colored worlds.
Bottom Line: Cheaper than actual coffee when it's on sale, highly recommended, double the friends, double the fun.