Both the original Zeo "alarm clock" and the newer Zeo Mobile for Android can be easily used and easily understood. They still work even though the company went out of business. However one caution: the Zeo's are very dependent on the sensors on their headbands, and those sensors do degrade over time. If you purchase a truly old and well used one, its headband sensors may be shot. If you buy a Zeo that was sold earlier this year, then their headbands are likely to have some good utility left. There are sites that describe how to replace the sensors yourself, and there has been some talk about some private individuals that may (or may not) sell replacement headbands with replaced sensor pads.
There is a community of about 2,500 very recent users of the Zeo Mobile, though I suspect 85% of them have stopped using the Zeo. Basically, if you do not plan to experiment and alter factors that can affect your sleep, getting a sleep measurement "score" is not really of much value long term. The value of the Zeo is to see some objective measurable results of seeking to change factors that influence your sleep quality.