Has anyone used the new Metro from IST to Old Town Istanbul yet?

Unfortunately, there is no Metro from the airport to the old city. That Metro line (M11) terminates at Gayrettepe, which is nowhere near your destination; it's on the north side of the Golden Horn. From Gayrettepe you could switch to Metro Line M2, which gets you across the Golden Horn but still probably quite some distance on traffic-clogged streets from your hotel. And changing subway lines is easier said than done. In Istanbul there's not usually a direct, underground connection. At Gayrettepe, you have to completely exit the system, go up to ground level, find the M2 station and re-enter the Metro. Google Maps says to allow 11 minutes for that walk, so it's obviously not just a matter of crossing a street, and I don't know how clear it will be which direction you need to head in to get to the M2 Gayrattepe station. (Be sure you have an electronic map on your phone so you can figure it out.)

You can go to Google Maps (or download the CityMapper app) and plot a route from the airport to your precise hotel location in Sultanahmet to see what I mean. To my knowledge, the only way to get to the tourist-hotel zone in the old city without needing to transfer twice is to take a taxi or cobble together a one-transfer route involving at least one bus.

I was in Istanbul as recently as last October. I believe you'll still need an IstanbulKart to take the Metro, and those machines can be tricky to use even when you aren't sleep-deprived and jetlagged. I wouldn't count on having an easy time of it if you try to get an IstanbulKart at the airport, unless there's a staffed counter where you can buy one (which there may be), though I hope the machines at the airport are in decent working order (not always the case with the machines downtown). The initial card comes pre-loaded with enough money to cover more than the cost of two Metro rides (no transfers, to you pay for each ride).

Given the complexities involved in using public transportation into the city, I urge you to go the taxi route. Your hotel may have a well-priced arrangement. I believe someone mentioned a figure between 30 and 35 euros on this forum last year. I would consider that a bargain. Getting a taxi from the queue at the airport might or might not be less expensive.

Even if you take my advice about the trip in from the airport, you will almost certain need an IstanbulKart at some point. Take advantage of any opportunity you encounter to acquire one without having to use an IstanbulKart machine (though if you need to reload the card, I don't think the machines can be avoided). The tourist office sells them, as do some little corner stores (probably at a modest surcharge). Maybe some hotels sell cards to their guests.

For others reading this: If you're staying longer in Istanbul and have decided to stay in the Taksim Square/Istiklal Caddesi area, you may have a somewhat easier route from the airport, depending on how far your hotel is from certain bus routes. But for a shortish trip, staying in the old city is a lot better for sightseeing purposes.

I haven't gone yet, but I have watched plenty of you tube videos in preparation for my trip next fall. You can find numerous videos, including those that show the walk between metro stations that you have to use to get to the other line mentioned above. It looked like a fairly long walk, but there was a pretty good crowd of people taking the same route so it would be unlikely that you would get lost. Just follow the luggage........

I found that a coach that leaves from the airport and goes to Taksim Square stops directly outside my hotel. I plan on using that for my transport in from the airport. If that plan falls apart, then a taxi.

I'd second the suggestion to plot out the directions/route using Google maps or CityMapper. In Google maps you can change the time of day to show transit options and predicted traffic. As a test, I put in this Tuesday, 5:30 p.m. departure from IST airport to the hotel I stayed at in Sultanahmet. For driving, it shows times from 50 minutes to 1 hour 20 minutes (my experience was just over an hour). Transit options are between 1.75 and 2.5 hours travel time - the options are two buses (plus a walk), bus and tram (plus a walk), and everything with the metro includes at least one bus and tram ride (plus a walk).

I arrived in Istanbul on a weekday about the same time as your scheduled flight. I used Welcome Pickups to get to my hotel - a flat rate for a driver paid ahead of time. A taxi might have cost a bit less, but since there was traffic I'm not sure how much I could have saved, though my driver said traffic wasn't "that bad" compared to usual. With the pre-booked service I had the convenience of direct communication with my driver, no worries about my personal belongings on public transit, and I was deposited directly at the door of my hotel. After an extremely long travel day, it was worth every penny to me.

Thank you all for your replies, I value your perspectives.
I'll be booking the Welcome Pickups, to simplify things and leave the Metro for another day.

Happy Travels!

Good choice! Back in November I did similar research regarding the metro and other transportation options and decided on Welcome Pickups as well. They were waiting for us outside the terminal and took us to Sultanahmet with efficiency.

You have made a very smart decision.

For others reading this who may be staying around Taksim Square or Istiklal Caddesi rather than in the Old City: You may have a less expensive option that is workable for you. It's staying in (most parts of) the Old City that complicates airport transportation.