I use Instapaper all the time. When I have 5 minutes before a meeting at work or early in the morning before I leave the house. The main thing I do is when I only have a small amount of time, I scan my normal sites for interesting articles and use the "Read Later" bookmarklet to add it to my reading list. I may add 20 things to the list in a quick 5 minute scan of HN, ESPN, Ars, & my email with links from friends of things like "gotta read this" or the like. Then when I have a bunch of free time (usually at night before bed), I sit down at my computer (or iPhone/iPad) and read through things I have saved. The text-only view on the iPhone and iPad is great.

I guess if you regularly have large chunks of time throughout a day, you would not need Instapaper. But if you are regularly running around all day with little to no downtime, it is nice to be able to sit down to one place and read through the days material.