I was contacted two days after applying online by a recruiter. After an initial phone screen with the recruiter (easy, but thorough) I was invited for an in-person interview the following week. You'll need to complete a pre-formatted cover letter asking why you want to work at ADP beforehand. The interview consisted of an hour-long meeting with the Sales Executive, very friendly and professional, where I was asked about my background in detail, why ADP, my general sales abilities and strengths/weaknesses, etc. I then spoke for 30 minutes with the VP of Sales, and after, with another manager in Major Accounts.
Those I spoke with were extremely courteous and displayed a genuine interest in my background. They seek candidates of sound character and believe sales acumen can be built on top of that, so be sure to demonstrate a solid foundation of character, personality, and confidence before elaborating on specific sales skills.
As others here have said, one crucial thing to know is that they often don't have a vacant position immediately available and you'll be placed on the bench after receiving an offer. The waiting period can range from 2 weeks to 2 months and is largely dependent on the needs of the office where you're applying. This was the only downside to an otherwise very positive and professional interview experience.