Re: Usage ?
I use ESE a lot to create mobile applications and right now am writing a WinRT/UWP component library DLL to allow interfacing between UWP apps and ESE. As a database, it's fast, super reliable, ACID, and free given it ships with every Windows release (ESENT.DLL). It's best for fixed database designs without the need for an ad-hoc query capability since there's no query language facility. Essentially is a very good ISAM database engine with some really cool features like "tagged" columns that take up no room if left empty. I write in C++, but there's a well-documented C# API as well.
ESE is fundamental to Active Directory and a host of other Microsoft built-in functions, so it's well maintained and continuously tweaked. If you have a fixed schema with a manageable number of tables and few complex joins, it's a viable alternative to MySQL, SQLIte, Mongo, and other database engines.
And a shout-out to Microsoft for posting the source. I've been waiting for *years* to get a better insight into the code behind the API.