I find it’s great for some kinds of work and terrible for others. I use it a lot when I’m researching and need to be just reading stuff (a task that is easy for me to distract myself from or get sleepy while doing) and I use it when sending emails (same problem, though less of the sleepies). The walking desk keeps me focused and awake then.
But it’s awful for writing. And even worse for organizing information into an outline as part of the pre-writing process. So I have a sit-down desk, too.