Full disclosure: I work for LEAD Technologies, but since you asked for advice, I hope you don't mind us chiming in.
Our toolkit has a strong OMR engine that is easy to implement and supports many programming interface. The toolkit comes with OMR demos that can help you test the OMR engine.
We have published a white paper article on CodeProject specifically dealing with OMR to help customers leverage our toolkit support:
Creating and Processing OMR Forms with LEADTOOLS
I would also recommend evaluating the customer support department as well as the product itself. While a great product is essential, having a capable support team to help back the product is also important to assist when issues come up. Great support can be the difference between being able to quickly fix problems and continue moving forward with development or getting stuck on a problem for extended period of time. They should also help point you in the right direction from the start so you can begin to rapidly develop your proof-of-concept application.
If you are interested, you can try the free 60-day fully functional evaluation version of the toolkit before buying it. Everything is unlocked by default, so you can check everything out.
If you have questions, you can contact the support team through live chat or email. Also, in the evaluation period, you can contact the support by phone for free.