How reliable is the Powershift transmission?

Hard pass. Nothing but problems. I would not buy it again.
There was a class action lawwsuit on the tranmission in fact.
We still have outs but the Powershift transmission clutch has been replaced 3 times.

I wouldn't buy another one unless it was VERY cheap. I mean CRAZY cheap. In my experience it's been unreliable, and unpleasent to drive.

The Fiesta is a good car, but the PowerShift transmission, particularly prior to 2015/2016 has been a problem. It's not even clear that a car that's been fixed has a proper long-term fix. I personally would skip a car that has the PowerShift transmission to save yourself repair hassles and an unhappy ownership experience. This coming from a 3-car Ford guy.

Summary:

I think if the one you are looking at drives fine, and has not had any transmission work, you are probably good to go. I would be very cautious if any transmission repairs have been attempted.

More details of my experience:

My 2011 Fiesta transmission worked great until at last year at 95,000 the car was hit in the front, and I had to replace the driver's side front wheel and some gear box assembly inside the transmission. This required they crack open the transmission. About two weeks later they then replaced the clutch pack under warranty at that time because after the gear box repair the car was vibrating excessively at idle though it drove fine.

A year and 20,000 miles later (115,000 miles total) I am experiencing some shudder/judder on first to second shift, though on majority of days it is fine. I may have to repair it again but I will use a different Ford dealer as the one I used has worked on it twice and introduced new issues each time. I have talked to one mechanic familiar with this transmission and he told me that it requires some skill and attention to detail to fix this transmission. For now I am living with it.

When test driving it, do some light throttle acceleration, in my experience that is when the shudder is the worst. Aggressive driving seems to mask it.

My brother-in-law has a Focus with same transmission, no problems. It seems if you get a good one, the transmission works great. Nevertheless, If I had to it over again I would have gotten the manual.

I've researched the manual transmission on the Fiesta and I have not found any problems with it. The automatic, on the other hand, is one of the worst I've seen. You should be fine with the manual, not the automatic.

take your money and run, far away!
known issues with these in all fords.
a friend has had her trans replaced twice already under warranty, and will be out of warranty soon. She still owes on her car loan, and feels like shes trapped in a clock-ticking time-bomb.

Keep in mind that Ford has extended the warranty for the transmission in all affected models/cars, so don't freak out if you think your warranty is about to expire. From the notice Ford sent:

This program extends the coverage of the DPS6 Transmission input shaft seals, clutch and transmission software calibration to seven (7) years of service or 100,000 miles from the warranty start date of the vehicle, whichever occurs first. Coverage is automatically transferred to subsequent owners.