I've carried one for years.
Fabulous in continuous rain; I wear my rain jacket with the hood down and the zipper down at least halfway. My head stays much cooler, I sweat a lot less, and my torso stays well vented and dry.
Fabulous in the desert sun. Makes a greeat overhead solar shield when stopped during the heat of the day. I recline on my CC pad with my umbrella suspended overhead. I added loops of cord to the tip and to the bottom of the handle. That way, I can tie the umbrella overhead and adjust the angle by moving the staked line on the handle as the sun moves overhead.
Great for high altitude sun protection while hiking.
The umbrella's greatest enemy is a sudden wind gust. I had the canopy of my GoLite umbrella stripped away in a flash just north of Mojave. When it gets REALLY windy, I either put the umbrella away or hold it in front of my body with just my eyes and forehead above the canopy. Caveat: Walk much slower than normal this way as you can't really see what's directly below and in front of you.
I've had them rigged to my pack so as to be able to use trekking poles too, but found that more hassle than benefit. It's too easy to break a support rib taking the pack on and off and it's difficult to dodge overhead branches, pass under logs, etc. I'll stick to handheld. YMMV