This review is by Kalina Wilson, who can also be found at geminica.com.
Noodler's Flexible Nib Piston-Fill Fountain Pen is an exciting product unique to its price range. It costs approximately $14 plus shipping from online retailers - if you can find it in stock. Both Goulet Pens and I Sell Pens are out of stock at the time of writing but will put you on an email re-stock notification list.
Besides the clear demonstrator model shown, it also comes in various colors.
In short, this pen is a little bit wonderful and a little bit a pain in the neck.
The body is simple and sturdy, certainly aimed more towards functionality than beauty. The piston-fill mechanism is easy to use and loads up a fair amount of ink. The nib leaks a little bit while not in use (note the splatter visible in the photo of the capped pen), but the cap seals well so there have been no accidents. The tip can draw a very fine line all the way up to a super bold line, and man is it fun to use for making fancy letters.
The drawback of this pen is that there can be flow issues. Even after washing the entire pen out multiple times with soap and warm water, it was a struggle getting it to write consistently. This sample was made with J. Herbin's Poussiere de Lune, and I got similar results using Noodler's Antietam. Note that some of the letters just... peter out.
This may partially be a matter of getting accustomed to using a flex pen, which requires some care to be taken with angle and speed. After pages and pages of tests, I did manage to do the animal sketches below using Noodler's Antietam without quite so much trouble as is evidenced by the above writing sample.
After a lot of efforts to make this pen behave with the J. Herbin and Noodler's inks, I obtained Waterman's South Seas Blue ink, which is recommended as one of the most free flowing inks around. It did seem to help and enabled some casual sketching on the bus at something close to my regular speed. Note that because this pen can lay down a broad line, it's especially susceptible to smearing.
After the above sketches I switched back to Antietam to confirm that what I consider a "regular" flowing ink has problems in this pen, and it was absolutely true - the sketch on the right was laborious to make. I switched back to South Seas Blue to produce the small drawing below, and perhaps you'll see from the sketchiness of that drawing that I was moving much more quickly and it basically worked, though even then the pen ran dry a couple of times. If your hand moves more slowly than mine as you sketch or write, you'll probably have an easier time of it, but it's still going to be a good idea to use an ink that is reputed to be fast flowing.
One other thing about this pen - it smells terrible, straight out of the box. Before you use it, take it apart (remove the nib and the plastic feed behind the nib, and unscrew the body parts) and give all the pieces a good, thorough wash with warm water and soap.
The bright side of the Noodler's Flex is that the variable width of the line gives a very distinctive look to any drawing made with it, and that's a lot of fun. With an ink that shades, the results are reminiscent of bamboo pen drawings but with a more precise minimum width. This is a perfect pen for showing off a beautiful ink.
When you want a smooth, reliable line you can just go to town with, this pen isn't the one to grab - but if you want an intriguing style that will give your sketches or writing a different look than you've been able to generate before and you don't mind working slowly, the Noodler's flex will provide new and unique opportunities.
Note: If you own a Noodler's flex pen and don't have the flow problems I've talked about, please post a comment and let us know! Maybe I got a bad nib; maybe there are a lot of other inks that will work well; I'd love to hear more happy stories about this pen especially regarding using it for drawing.