kiltedcelt, Not on trekking bars, but I've used fixed (Syntace C2) and non fixed (Airstrike) aero bars. The Syntace bars are slightly more comfortable than the Airstrikes, but in the end I didn't like losing the area on the tops. The Airstrikes also come in Z back (pads slightly behind the tops) and non Z back (pads inline with the tops) to tailor where your forearms rest.
For the recreational rider looking for an alternate hand position I like the Airstrikes.
Brad