Does azure search uses Lucene as search engine?
Azure Search is Microsoft's response to providing a cloud based indexer. It uses Elastic Search as the underlying driver, but the Elastic Search engine is not directly accessible. Currently Azure Search is only accessible using RESTful Web services or the Azure Resource Manager control panel.
I assume switching from SOLR to Azure Search will be easy i.e. no code
change (just use content search provider)?
As for the Sitecore provider, it will hook into Sitecore in a similar fashion to Solr and utilize all of the same Sitecore ContentSearch api.
My initial thoughts are that it's possibly viable so long as all of your Sitecore roles are in Azure and in the same Resource group as well as same virtual network.
Any other use or connection and the latency makes indexing so slow that it's not recommended. To give you an idea, it took approximately 9 minutes to index 1000 items from local laptop to Azure Search.
For a full cloud implementation though it is very promising.
Has anyone played with Azure Search?
Jamie Scott (Slalom) wrote one of the original implementations that works with Sitecore 8.1. You can find his Github repo here: Sitecore Azure Search Provider for Sitecore 8.1.
Here is a link to the LinkedIn Blog post he wrote about it.
Does azure search has concept of Cores (similar to Solr) so that we
can patch per environment?
Azure Search uses indexes as the primary collection. If you need to use different indexes for a different environment you can setup a different service connection.