I switched to OptimusFutures recently and completed integration with R-API Plus .NET/C# API, thought of sharing my experiences that could be useful for others.
MattZ was very helpful in getting access to Rithmic API immediately with accounts on Rithmic Test and Live Trading platforms. I received API documentation and sample code from Rithmic. API itself is very well designed and well documented. Sample code provided with documentation is fully working examples for both market data and placing orders, they can help you get started right away. Other products with similar API access provide synchronous/real-time response for order placement but we have to keep checking over and over when there is action on resting orders like limit and stop orders. But with Rithmic, their callback mechanism calls our code in real-time when there is action on resting orders. We don’t have to fry our cpu for hours trying check the status of resting orders once every 500 milliseconds.