Firstly, it's open source, that's a big plus to me, then It's fully customizable, it's stable and reliable enough to be deployed on production, though there are some bugs here and there but it's minimal and most of the time resolvable, You can ask help from the OpenStack community here and/or in the Mailing List, the community is huge. I've deployed and used it in production since 2014.

i search a lot and the answer is nothing

some one say it has api or it is free but those thing nothing important beacause esxi has api and so simple