Thanks for the Questions:
How often do you use the service every month?
Looking back at the last month I used it for 22 flight segments (all intra-California, none to Vegas). Most of the flights are from SoCal to the Bay Area. I'd say that might be an average month for me. I did see SurfAir shared an info-graphic that the
#1 user booked 801 flights, presumably over their lifetime of membership.
* Do you use it as a commuting tool (fly out in the morning, back in your own bed at night)?
That's my ideal day. Up in the morning and back in late afternoon-early evening. I use it mainly as a commuting tool but when work requires me to travel I use SurfAir so a couple of times a month there is more of a large loop through the state that has me hopping between locations and overnighting between the flight days in a hotel.
* How full are your flights? How far in advance are popular routes sold out?
It depends on the route. There seems to be a cycle to the crowdedness of the flights. The SurfAir configured aircraft (Pilatus PC-12 NG) carries at most 8 passengers. Typically on my flights due to the short runway at San Carlos SQL they may limit the passengers to 7.
Generally I've observed that once the flights start to get full they'll assign the next new aircraft that is being delivered to that route and add another flight time in the schedule. That generally reduces the congestion of fully booked flights and gives the added benefit of another flight time being available for flexibility in scheduling.
The down side is that there needs to be another aircraft delivery and your congest route has to the winner of the new aircraft. More or less SurfAir has done a good job managing this but there are not as many new aircrafts being added so this will be interesting to watch, especially as summer draws to an end.
With rare exception all flights more than 1 week out are open but if you're booking with in 7 days YMMV. Looking at a popular SQL-SBA route for this week:
Most days and flight windows are open. The shaded in circles indicate that flights are full in that time widow. For example the 3 flights to SBA on Thursday leaving after 5:00 where all full. This is probably typically of the SoCal<->NorCal flights that are full on Mondays and Thursdays. It's rare nowadays that you'll have the entire plane to yourself but that did just happen to be this month.
* What are some annoyances that you experience regularly?
As an airline goes there are not that many, and none that are regular. Even then, the issues that arise are just first-world problems. I've certainly shared a lot of feedback to SurfAir on items they can improve but overall the experience has been exceptional. Not only do the concierges know you by name but so do the pilots. Never any airport hassles other than just the traffic trying to get to the airport.
One thing that is hard to get use to flying commercial when I can't fly SurfAir. As a frequent flyer (most with WN and UA) you start to notice the time that a commercial airport takes. I've gotten use to pulling into the parking lot at T-minus 20 minutes (For surf air that's be considered on-time or perhaps a little early for the flight.) Also the time to board the aircraft to taxing with the props spinning is usually less than 5 minutes.