I had a Sense for about a week. TL;DR: looks pretty, but it's also pretty useless.
The Sense is an environmental sensor and sleep monitor. You attach a coin-sized sensor to your pillow for sleep monitoring (based on motion, so not that accurate), and the base station monitors the environment for light, sounds, and "air quality". What exactly air quality is, I wasn't able to find out (Volatile Organic Compounds, apparently), but it had a percentage value (percentage of what?) around 45, which was "good".
The base station is a nicely designed spherical orb that can light up in a few different colors, and it can also sound a gentle alarm. When you wake up, you can get a report about the quality of your sleep.
The big problems with the Sense are as follows:
The light and noise sensors are obvious, wrong, and pointless. For example, the TV was on and the Sense glowed green, indicating that the sound is soft enough (under 60dB - the volume of a normal conversation) to sleep. Well, let me tell you - people why buy sleep monitors tend NOT to be able to sleep while there's a conversation going on. They're more likely to use earplugs. The light sensor was similarly relaxed, while it's been well documented that you'd better have blackout curtains if you're a sensitive sleeper.
If the noise sensor detects some noise, the Sense will tell you in the morning that there was some noise, but it won't tell you when, or what it was. This makes pinpointing potentially disruptive sounds an exercise in frustration and futility. Again, just use earplugs.
The coin-shaped motion sensor should be attached to a somewhat specific spot on your pillow. It's unclear how you can recharge the sensor, what the battery life is, if it's disposable. Also, if you roll off the pillow, and possibly also if you sleep far enough from the sensor side, good luck.
To sum up, the Sense is a modern, aesthetically pleasing device, maybe even a conversation piece, but if you're serious about monitoring your sleep, stick to the Emfit QS, which has recently updated their sleep recognition algorithms.