Why do you need adjustable end links? Well, they allow you to remove any preload from the sway bar. They're not necessarily required with every aftermarket bar, but they are the last step in corner-weighting the car to ensure it's perfectly neutral.
The weakest point in adjustable end links is usually the rod end (or "Heim joint") at the ends. Rod ends allow for a good range of motion and they look like awesome race car parts, but they wear over time. The speed at which they wear depends on the quality of the bearing and the environment in which you live. Some inexpensive ones have a lifespan of 6 months. There's a reason you'll never see rod ends on production cars.
This is a custom end link made for us. No fragile or wear-prone heim joints here, we specified a rubber bushing at the bottom like the factory did so that it plays well with the stock mounting point. At the sway bar end, we went to a ball joint for maximum freedom of movement and OE-levels of durability.
The end link can be adjusted turnbuckle style, without disconnecting either end. Minimum length is approximately 4.25" center to center.
Sold in pairs, which will cover one sway bar. Two pairs are needed for a complete car.