I have been using the same Mako saddles for almost 15 years, for two 17' sea kayaks, over extensive distances. The Mako saddles perform well and they last ... and last. For a lower car with no high-level spoiler, you don't need rollers -- the boat will slide up into the saddles quite easily. The strap system is idiot-proof -- you just can't put the straps on upside down or backwards.
These saddles do have a bit of a flex to them, which is un-nerving if you aren't used to it. But I discovered (the hard way) that the alternative -- a hard, rigid tie-down with no flex -- doesn't work well in high cross-winds. Once, when car-topping the boats in gale-force winds on the NY State Thruway, the wind blew the car about a foot sideways. Now, with the Mako saddles, the flex absorbs cross-winds, rather than the car.