Absolutely critical!
In short, I don't go it a day without Dossier. I use it for keeping a journal for work, noting what I've done each day for each project I'm working on. I wanted something for which I could view or search everything cronologically, or else by project. Also, the ability to paste in pictures/plots/etc. was essential. Dossier more than fills this role, with additional features like smart folders which are also helpful. Since I'm writing every day and often pasting pictures, there's quite a lot of stored information. Despite this, Dossier launches quickly and without problems.
Support is also exceptional! A couple strange behaviors slipped in with the upgrade from 1.? to 2.0, which I emailed the developer about. Each time, the email was acknowledged almost immediately and fixed within a couple hours. Version 2.0.1 contains all the fixes, and I haven't been able to find anything wrong with it.
Truly great stuff, I recommend it highly!