There are plenty of reviews about the Desiderata out there, but most of them are from the POV of a copperplate calligrapher, which is this pen's intended use. There are few reviews that focus on drawing, which is a shame, because I think this is a nice, and also an overlooked art tool. In case you haven't heard, these are made by hand by a fine fellow named Pierre Miller in Chicago, and they take disposable Zebra G manga dip nibs. I won't re-iterate the information already available on Pierre's site, but instead I'll dive right into the stuff about drawing, that isn't widely available.
Below is a sketch test with Platinum Carbon Black in an eyedropper-filled Desiderata shorty, with some white grease pencil for the highlights, and red marker in the eyes. The sketch is on gray cardstock. Initial results are very favorable. I have read one comment online claiming that, in sac-filler configuration, the pen does not flow enough to keep up with the demands of drawing. My pen does not accept a sac, so I cannot confirm or deny that claim, but an eyedropper-filler provides plenty of fluid pressure, and easily provides enough flow to keep up even with my frenetic drawing pace. The pen did not railroad at all, nor did it exhibit the belching and vomiting mishaps that eyedropper pens are known for. Maybe that's just because, if you draw quickly like I do, the heavy demand tends to stay ahead of the belching. But one way or the other, I did not have the problem.
That being said, I'm not ready to junk my dip pens just yet: the Desiderata only accepts the Zebra G, and will not feed inks meant for dip pens such as Speedball Super Black. There are other nibs that I need to use sometimes for different effects, and sometimes I need to use inks that won't work in the Desiderata, and let's not forget I still need a brush for big black areas. But the Desiderata is nevertheless poised to become the workhorse of my comics inking. In comics, speed is everything, because you have to draw so much to finish a book. If you don't have to keep stopping to dip, you can go so much faster. You can also get into a mental flow that is harder to get into when you have to keep dipping, which is why Sergio Aragones has drawn all his strips with a fountain pen for the past 50 or so years. The other tools are useful, but they will be held in reserve to do the things the Desiderata can't do. The Desiderata is also going to cause all my other fountain pens to get demoted from cafe-sketchers to mere writers. For sketching on location, where carrying an inkwell is inconvenient or impossible, the Desiderata is the best pen I've ever used.
The biggest technical hassle you are going to run into is nib cleanliness. If you're used to dip pens, you're no doubt familiar with the machine oil with which new nibs are coated, to protect them from rust. You also know that this stuff has to be cleaned off. Well, a nib that's clean enough to dip into a pot of bone glue and lampblack, is not clean enough to use in a fountain pen. I myself learned that I didn't even know what clean was. If there is any trace of this protectant on the nib, the ink will find any other way to flow rather than traversing a hydrophobic surface, if it flows at all. Pierre has videos posted about how to clean a new nib, and how to start the flow for the first time. Watch them. Then watch them again. The devil is in the details, as our pal Old Scratch so kindly points out in the sketch below.
Oh, one more thing; make sure you carry a full Desiderata with the tip up. The feed channel is necessarily very large, to accommodate the flow demands of a flexible nib. Also, the sac filler pens can be converted to eyedropper by removing the sac. That's all in the instructions. So that's the review. Happy sketching!