According to ReadWrite, it has an unimpressive battery life, a weak speaker, and is closer to being a smartphone accessory than a smartwatch. The Verge says that the software leaves a lot to be desired:
First and foremost is the speed and intuitiveness of the user interface — or rather, the lack thereof. There's a tangible lag to anything you do with the Gear, while the swipe gestures are hard to figure out and do different things depending on where you are in the menus.
But that's not the most useful review so far.
TUAW also reported on this story, and they took quite the different approach. Rather than criticize the Samsung Galaxy Gear directly, they compared it to the 2010 iPod Nano. They even put together a handy little chart which showed the basic abilities of each device.
I embedded the chart at the end of the post, but the tl;dr version is that the iPod Nano is a better smartwatch than the Galaxy Gear. It costs half as much, is more capable, has better battery life, and doesn't need to be paired to a smartphone.
Yes, I know that some will say that it's normal for a smartwatch to be paired with a smartphone, but IMO any device that can't operate independently is not smart enough to be called a smartwatch. It's a BT-equipped smartphone accessory strapped to your wrist, not a smartwatch.