Greetings,
When the Control Engineering "Engineer's Choice Awards" come out next year you will likely see Kepware's OPC server receive recognition for the 5th year in a row. The voting is not a scientific poll, but based on reader preference. Sometimes preference speaks volumes over a scientifically engineered poll with carefully crafted questions eliminating bias and generating a consensus within a certain margin of error.
With all that being said, Kepware's product is an industry leader for OPC servers. So much so that even some of the larger HMI client software vendors have incorporated an OEM version into their software.
From personal experience, when people who have worked with the product are asked about it, they offer praise for its reliability. One person told me "It Just Works." Nothing glamorous.
Kepware continues to build functionality into the product with this past release addressing data through the introduction of two new plug-ins - Local Historian and IDF for Splunk.
So, even though their OPC server isn't the only one, it is recognized throughout various industries as the standard.
Which now leads to the question - Why do you ask?
Disclosure - ESI is an authorized distributor for Kepware.