I got the game off a store that broke street date and I must say, it's awesome!
Even without the ability to create your own stuff, the game is pretty fun. It's a simple platformer at heart, but the realistic physics engine adds a lot to the experience. A lot of effort has been put into level design too, there are many moments where I thought "wow, that's a neat idea/puzzle!". Gameplay gets harder fast (I suck at games), and the controls are a bit weird, but the game is definitely worth its price. I liked the multiplayer mode with my brother (can't try it online until the official release date), especially with the special areas designed for player cooperation, and I loved how there are tons of bonus items to miss (on average I beat levels with 50% items collected the first time). It's by no mean a game to make people buy a PS3 or change the way we think about video games, but it's a great game to add to PS3's poor library.
Creating your own levels is time consuming and not as intuitive as it sounds. The interface is simple, and there are some advanced tools to create more complex levels (think literal RPG Maker switches and really simple conditional branches), but you'll quickly hit the limit and have people trying to go beyond them like that calculator demo (man, that must've taken forever). I guess it'll be nice to have access to user generated levels when they're available, even if you don't make your own. One cool thing is that you can set some rewards using items you created (say you created a car), and then other players will get this item once they beat the level. You can choose to disallow other people from sharing your items with others, thereby forcing everyone to play your level to get a particular item (e.g. someone made a working Metal Gear REX).
So far I enjoyed creating weird contraptions more than creating actual levels. For example, I made a simple device that throws an infinite number of gigantic fish with wheels and a rocket engine in random directions, and soon enough the entire level was a pyramid of huge rocket-propelled fish on wheels bouncing around and squeezing each other, causing the PS3 to lag a bit. It doesn't sound as cool in words, but it was pretty fun... creating something and playing with the physics engine.
But yeah, it's a fun game. I wouldn't give it 10/10, but it's still one of the best games I've played in a while, and everyone should give it a try.
Edit: I'd also like to mention that there are some bugs in the game. I thought it was only my copy but my friend and some beta forum posts confirmed it. Sometimes the game just freezes and you're forced to restart, the PS button almost never works for me (freezes), and it's possible to get stuck without being able to do anything but restart the game (even with the teleport-to-last-checkpoint) option). It's not really frequent but I guess they didn't fix all of the beta bugs.