I applied for the position online and heard back months later, when I was asked for a phone interview. I did two phone interviews, each a month apart, with different Groupon designers, before being scheduled for an in-person meeting. However, upon getting to my interview early, the building was locked and I had no way of reaching the recruiter who'd been acting as liaison (she wasn't picking up the phone or responding to my email). I had to wait outside until another employee arrived and then ask for help in tracking down my contacts. Eventually we got things sorted out and I was there for about 4-5 hours, talking with different designers and teams and getting moved from one meeting room to another (including one without any chairs at all). Everything felt rather disorganized, although everybody I talked to was very personable and excited about working at Groupon.
Afterwards, I got a request to complete a design exercise (which is standard in their process and pretty normal in an interview). I completed it, sent notes explaining my thought process, and didn't hear back for about 2 more weeks, at which point I got a polite rejection. When I asked if I could possibly get any feedback for future reference, they never responded.
Overall, the in-person interview experience was positive, but the lack of communication and disorganization were huge negatives, so I'd rate the process in between neutral to slightly negative.