I love my Easyboot gloves, but I do most of my own trimming, and I trim nearly every time I ride. Some horses still fit their boots ok with several weeks between trims, though. My neighbor trims her Peruvians every 6 weeks and doesn't have too much trouble with Easyboots, though they occasionally come off in bad mud. I think her horses also are more prone to losing them because of their gait and strange pastern angle as much as due to trimming schedule. I have never lost a boot on my Welsh mare, and I've only had one come off of my QH after she jumped an embankment and crossed a bunch of deep mud. Both horses' boots fit the first time, but I measured carefully, and it was not my first time booting horses. One comment on booting: if your horse is transitioning out of shoes, and/or is not getting a barefoot trim/used to being barefoot, boots are not a magic bullet. They help, but the horse will still be a little ouchie crossing gravel and rocky terrain for several months. You can add pads for comfort, but ultimately, it seems to take around 6 months with correct trimming to completely transition. I still find them to work better than shoes for my horses, and they are SO much cheaper.
If you've never fit a horse in boots before, I recommend the fit kit Easyboot. They send you boots to try on, you pick a size, order your own, and send the kit back.