We have a large library of OPC servers we have used. We try NOT to recommend OPC servers to the customers for the following reasons:

• Machine Specific – although OPC goes on about how it is universal etc. Most opc servers are tailored to work better with certain machines. We leave it up to the hardware vendors to suggest this or allow the customer to do this on their own.
• The OPC servers are installed, maintained and configured by the customer (or their sub-contractors).  We have no control over this.

We can for sure share our experiences and suggestions.