Very good device for monitoring heart rate and sleep. The heart rate is out by about ~5%, but that is always going to be the case with this type of HR monitor. The sleep tracker works well and will definitely indicate if you have an issue with sleep or not. The app is good for logging, although the bluetooth sync can be a bit slow at times.
The only downside is that he strap will fail within a year if it is used a lot. This is because the connector they use for the strap is very cheap and will tear from the silicon strap and then break. However you can pick up 3rd party straps for very cheap which seem to be higher quality.