My testimonial for Kepware -
I was recently encouraged by management to try the Matrikon OPC server with the Modbus driver. Their package includes an OPC test client. I downloaded it from the Matrikon web site.
After installing the package and dinking with it for several hours, I was stuck without having any clue how to configure it.
I called Matrikon and spent about 45 minutes trying to understand a non-native-English speaking tech support guy. He kept telling me that the Matrikon OPC client had be configured to talk to the field device, but he couldn't tell me where in the test client that configuration took place. Specifically, he couldn't tell me where the field device's IP address was specified - pretty basic communications stuff.
He finally dumped me telling me he'd email me an illustrated method for Modbus driver configuration.
He did send it, but his screen shots show things that are not on my version and are essentially 'random' screen shots with no context. Several attempts to re-contact him failed.
I grabbed another laptop with a 2 year old version of Kepware already installed on it and poked around. I couldn't remember how to make a new 'project'. I called Kepware and the guy told me to right click and select "x whatever". That's all I needed. I thanked him, hung up and had Kepware configured for the field device and talking within 10 minutes (it would have been 4 minutes but it took me about 6 minutes to re-discover where to byte swap the floating point). So in about 10 minutes, it was up and running, Modbus slave device, connection and the Kepware server/test client.
I'm sold on Kepware. (and I don't rep Kepware, I pay for it like everyone else)
Dan