Im based out of DEN now. I fly F9 regularly to commute to my residence near MSN (basically to just cut my lawn and check on my house). I always buy tickets at the counter, for future travel, to save the $23 CIC. Never had a problem but I always make sure I'm on the PNR even if I'm not traveling. If you're not on the itinerary then they'll refuse to sell you the ticket even if you then change your story say you will be traveling now. I literally just bought three r/t tix to MSN to watch the Pack vs Viqueens. I was the only one there and bought two additional tix for my wife and step kid (different last name). Zero wait at the F9 Cust Svc line too.
I HAVE tried to buy tix at the DTW counter and they played stupid games. Had me go to the back of the line (after waiting in line) and then when I was up front again they refused to sell me a SINGLE one-way ticket, for a later date, stating I had to "wait until all flights are boarded". This was at 11am. They said the supervisor won't be available until 1pm...
I grew up in metro Detroit. I attributed this customer service fault as "The Detroit Effect" or "Island Time" mentality. Denver F9 staff are 100x more respectful and professional.

Edit: I add myself to the PNR even if I'm not traveling as I buy a massive amount of travel when they do the cheap airfare. When you miraculously stumble across a $19 airfare, it waives the CIC even if you buy with the app. If it's $38 then they charge $10 CIC, etc. I obviously will buy in-person if it's worth saving the fee. Sometimes the Discount DEN savings is $20 or more l thus it isn't worth my time in most cases. Plan accordingly and you could save a fortune.