Absolutely talk to your core of most important customers but then recruit a core beta testing group from them and use them for your detailed feedback and user research.

I just completed a three week beta test with the Guardian Newspaper on their new mobile app (I didn't like all of it) but they sent us a questionnaire a week and I tracked my use of it and the functionality/features.

Remember, in terms of new business development, those beta testers are in all likelihood your first customers for the new service - work them hard, make them feel special and then use their feedback and recommendations to launch and market the finished item.

Happy to amplify further if you tell me more about your business industry / service & product.