Kaizer and I have been playing on Expert mode. I'm not one to judge because I haven't played through all the content since they first added hardmode so there's a lot more in it than just the 1.3 stuff. We also haven't gotten steam servers to work. I think I have an idea on that end, but we'll see.
It's tough to say if the new new content is worth it yet, since, y'kno, it takes to the end of the game to get access to it. We spent our first day just foraging and base building, so haven't run the new Eye of C'thulu yet. Some of the new normal enemies are cool, though. There's a purple snail that's basically sonic the hedgehog, spin dashes and homing leaps. There's a Granite Elemental which will float toward players until it's on top of you or takes a few hits, when it will close up and slam down like a boulder trap and wake up a few seconds later. Like I said though, most of the new stuff, like the lunar towers, won't come up until the endgame.
Probably the coolest part so far is that the villagers all have unique AI patterns instead of running and hiding. The guide shoots a bow, the demolitions expert throws grenades, and the nurse has an ample supply of potions, for example. While they have a tougher time for us on expert mode enemies, in the normal game they would probably be able to fend for themselves much better during goblin invasions and the like.
There's also a new material in the cavern layer, Marble, which creates it's own miniature biome when spawned that populates with Medusa, some powerful spear throwing Hoplites, and the poison-spewing bipedal salamanders. The marble itself is a really impressive-looking material and I personally like working with it. The base I'm working on in our world has a huge marble outer shell with a large dome and a columned entrance, similar to a stereotypical U.S. government building. If we play again today I might grab screenshots.
In the end, though, it's Terraria. If you enjoy it, you enjoy it. If you don't, you don't. I doubt anything in this update will change that too much.