have one - a GoLite. nice umbrella. bought b/c RayJ uses one and i was, at the time, reading his BB book. tried it. rarely use it.
1. to be really useful, since i use trekking poles 90+% of the time, i'd like to figure out how to reliably attach it to my pack (it would probably damage the pack in the wind) or pack straps. better yet, to my back - sorta' like the unit/clan/family flag of a Feudal Japanese Samurai or Ashigaru (lesser foot soldier). hmm...let's see, i should come up with my own clan "mon" for the umbrella - a pair of crossed trekking poles, perhaps? now...how to affix it to my back. Fornshell-san, tsunoru, kudasai.
2. for some rains it works fine - subject to it's primary limitation, at this time, viz. #1 immediately above.
3. when it comes down heavy or in torrents. it's little use except to keep my head dry which my poncho and OR Nimbus Sombrero do also.
4. wind driven rain - umbrella offers little protection. something is always exposed. umbrella actually works better under heavier tree cover (on trail - not bushwacking) - less wind & less rain.
5. 40mph gusts with T-storm activity. i don't use it when storm is nearby. if i'm not under heavy tree cover (happens once in a while) with the storm farther away, the winds alone often make it unusable. i'm fighting with it the whole time the wind is blowing hard. this is why i can't find a reliable method of affixing to my pack or pack straps. anything that can put up with 30+ minutes of 40mph gusts and 25mph wind is perhaps not a viable attachment option. an umbrella shouldn't be used in these conditions, IMHO. so if i also need other rain gear, why should i carry the umbrella?!!
6. i hike in the NE - forests/woods, trails both with lots of tree cover. <5% of the time i might be exposed to the sun, sometimes even less.
however, i can understand why someone in the SW might want to use one. you can't argue with RayJ's experience - he sure has enough of it.