Never had any problem with CBP when visiting my girlfriend last and this year. Questions ranged from none and just "you know the drill, right hand...." to "where and exactly when did you meet? what do you do for a living? what is your major? what do you intend to do afterwards?".
Been staying for like one week plus a few days each with a 1 to 2 month frequency.

However, if you cannot convince them or they are not convinced of you doing whatsoever and going back to your origin, they won't let you in - and in case of a VWP you also waive your right to appeal to that decision (compared to visas).
I got Global Entry the last time I was abroad and I'm going to use it for the first time in Miami at the end of June.