I have tried both NeoLoad and LoadRunner. We had more success with LoadRunner. The only problem was that we had to manipulate the LoadRunner configuration of the generators (including the machine we scripted with in vugen) to allow enough memory to handle the massive requests that Flex made. We were able to run far fewer vusers on generators that tests Flex than those that tested http since each vuser used more memory. It was about a 10:1 ratio or fewer.