We have just setup all our devices on a trial license on OpManager 7 and we're quiet happy with it. The new license form (per interface, we're only monitoring switcthes/routers/firewalls) makes OpManager 8 much more expensive.
The reason we tested OpManager 7 to start with was that it looked like a very affordable product but with 2000-3000 interfaces OpManager 8 does not.