I am actually finding Timeful a pretty brilliant addition to my GTD arsenal. Reason being, most of my days are self-directed--although I start a new job on Monday, so that will change. But right now, I need to work about 4-ish hours a day (at any point during the day) on my consulting job, and complete a number of academic tasks every day and still make sure other things happen like cleaning the house, doing laundry, making food so I don't starve, etc. I do all this work from home.
I use it the way one might use a focus list, which David has talked much more about in recent years than he used to. The brilliance in it for me is that I really have to think about how much time a particular thing is going to take. So when I'm choosing say, to focus on revising Chapter 2, reviewing abstracts, and doing some personal writing, I can look at that and realize I've committed six hours to that work.
My context lists decide what goes on my Timeful for any given day, and when something finishes faster or I need to change what's going on, I go straight back to my contexts. But I'm finding it useful to have a reminder that, hey, realistically, looking into this new CMS should take me about two hours, and then I need to hop on edits to the request for proposals, and then I need to call it quits for my job for the day and get back to work on my dissertation.