I've used Skiplagged as a resource when booking flights for my brothers. I use this technique twice a year for my brothers when booking our family vacation. I wouldn't book through Skiplagged.com. You should book direct on the airline's website. Best to book regular economy instead of basic economy. Also, you should only book one-way tickets when doing a hidden city ticket. The airlines can and have canceled the unused leg of a roundtrip ticket when you "skiplag". Make sure that you get there as boarding time starts so you are able to get your carry-on in the overhead. You can't do checked luggage obviously when doing hidden city ticket. Do not put in your frequent flyer number when you make the flight reservation. The point is to do this occasionally and to fly underneath the radar of the airlines. We have done this on Delta and American without any issues.
The heads of airlines are upset about hidden city ticketing, especially United, but it isn't illegal. United Airlines unsuccessfully sued the founder of Skiplagged.com. Courts ruled in his favor. It is definitely a loophole in the system that shouldn't be abused regularly by a customer.