I've used them quite a bit.

They are wider than drops, which is a plus sometimes, but can get in the way at others. I never really use the farthest position, just isn't very comfy and i feel like it makes my core muscles less useful compared to being in the drops on drop bars. The sides/hoods on trekking bars are just a hair wider than ideal for me, but still very useful and comfy. Not as aero though.

The biggest drawback i have found is that i spend most of my time on the hoods/sides and on the trekking bars that means coming up and back to the shifters and brakes. On drop my brakes are on the hoods, where i spend more time. If I'm on the tops I find it easier to move forward to the brakes than the other way around on Trekking bars.

Really easy and chap way to convert a flat bar bike to have more postions though. They definitely have their place, but I wouldn't build a bike around them.