My Defender is the older version with a cover over the home button. It was difficult at first pulling up the Control panel, but it got easier with experience. I rarely use the Control panel anyway. This older version is better at keeping dust and debris out.
Just got the OtterBox Defender for my iPhone 6, and I'm definitely having trouble bringing up the Control Panel. This may be amplified by the fact I also have the Zagg GLASS protector installed, but I'm not quite keen on the idea of removing either that or the OtterBox shield.
Had the same issue with the Griffin Survivor case I switched from, but eventually figured it out -- but with the OtterBox's hard plastic casing covering right up to the edges of the screen it's definitely going to be more difficult.
Did you find a specific method that works best for you? I seem to be able to *randomly* pull up the Panel if I just keep flicking at it from the "edge" of the Home/Touch ID button...