I've had Timbrens, and they are durable and do what they're advertised to do. That said, the unloaded spring rate is not nicely progressive, as you're making a sudden transition from disengaged to engaged. It can create an annoyingly jouncy ride.
HD trucks don't suffer from this as greatly as 1500 series trucks since the leaf spring pack is significantly stiffer and it takes a big bump to engage the Timbrens.
In a loaded situation the Timbrens will be engaged at all times and the ride is not objectionable. I think that lower Stableloads are slightly more effective at reducing sway, but for the price, the Timbrens are an excellent alternative.