I read a LOT on my phone. Sometimes I get where I'm going early or have a couple minutes so I whip out my phone, call up the kindle app or Google's Books app and read for a few minutes. It's great.
I think it's reducing my enjoyment of some books though. I've made an effort recently to change the way I read a book. I've got several books going all the time anyway so I'm reading one of them, Snuff, by Terry Pratchett, in big chunks. If I've got more than half an hour to read I go for Snuff. Anything less than that and I grab one of the other books. I really enjoy immersing myself in a world, into a book, and my theory was reading in the short fits and starts of the drive through line and during breaks at work was reducing my enjoyment of reading, or at least of the books I was reading.
So far I'm half way through Snuff and I'm enjoying that time reading more. I'm enjoying the book more. It feels more cohesive to me. I may have to get some short stories or online magazines to read during those short bursts in the future so an otherwise good book isn't hurt by the tiny chunks of time I manage to scrape together through a day to enjoy the book.