Do your research! Do due diligence by researching the company; they will ask you what you know about Ultimate Software. Be sure to mention more than just recent honors and awards; mention things like their unique benefits package, outreach, communities, HQ, and most importantly their software.
The process for interviewing for me was a little unique given I worked for one of their clients and had to get approval from my previous (then current) job's Human Resources department. So heads up, if work for one of their clients or competitors, be sure to keep that in mind as it will slow the process a bit. My recruiter was amazing. She made the experience very personal and comfortable.
I had a series of interviews/of the overall process. One phone interview with the recruiter, One assessment, One in-person "blitz" interview from one manager to the next, with a total of three managers. The assessment has many common sense and math-related questions, so be prepared.
One thing I questioned a little was that I was not the only one present for the interview, given they knocked out all possible interviews at once and their was a group of us waiting between each one. This took the spark and anonymity out of the experience a little because it was never mentioned, and I assumed I was going to be the only one their for that period of time.
Everyone was cordial, professional, and very nice. The experience, in general was very good. For me the process took a little over one month, which was okay for me given I already was working at the time.