I am looking for a solution for real-time data integration between few on-premise databases. There is no much transformation of data involved. I am evaluating various ESBs available. I am thinking that data integration using Azure Service Bus as quick to develop a solution. Is it advisable to use Azure service bus for integration of all on-premise databases?
If you have all your applications 'on prem', you introduce an extra risk by moving your integration layer to the cloud (suddenly your internet connection could bring down your integration layer) but the good news is that you can use Service Bus for Windows Server, that you run locally (even with Windows Azure pack!) The same programming model, similar messaging features, so that might be a good option.
Comparing with BizTalk... Service Bus is light weight, messaging only. BizTalk provides much more rich features (transformations, pipelines, BAM, Business Rules, adapters).
Good luck