The basic criteria are use case and performance. Use cases can vary from data center to branch office activities, and from deep application inspection to securing XML traffic. Performance requirements also have a large impact on which form factors to use where. We currently have deployment work going on in various industries including financial [FIX], retail [RFID], telecom, B2B, and others. Feel free to send specific requests to "ask-aon-pm" mailer list if you need more help.
Thanks for your interest!