(click the picture for a much larger/hi-res version.)
Here's a review of a nice little fountain pen I bought from DWL - well, actually, it's technically a ball-point piston-filler, but as you can see, I mostly use it now as a FP.
My first impressions of the pen seems to be echoed by a few people - the beauty of the little pen comes in that the ball-point unit can be easily swapped out with another points - the Esterbrook Renew-Points seem to be most commonly used, but apparently Venus/Osmoroid (basically the same as a Renew-Point, I believe), Sheaffer Cadet, and Pelikan M200 sections can also be put into the pen. The ball-point unit seems too large to fit in an Esterbrook - the little ball-point unit (as you can see from DWL's sale post) has two series of threads, and only the initial set goes in without difficulty. The second set doesn't seem to want to go in without force, and I don't want to damage my nice Esterbrook LJ as of now.
The pen's piston-fill system is also very nice; though a bit stiff (pardon my vagueness - my experience has only been with sac systems or otherwise cartridge/converter fillers) as compared to, say, a Cross piston converter, it works very nicely and draws up a good amount of ink versus regular cartridges. Using a syringe to fill directly, it seems to take up a bit less of the amount of a Waterman long-international cartridge. Very nice.
The ball-point nib unit seems a bit finicky in that it requires some "soaking" of the fibers to get it to go. Drawing circles and such may help, but repeatedly drawing/expelling/drawing ink seems to work better for the ball-point unit to start. When it does go, it seems to be very smooth for a ball-point at any angle. Though DWL mentioned that the ball-point action seems to be very scratchy when the pen is used perpendicular to the writing surface, it seems to glide on paper at any angle for me. A definite thumbs-up for this. The only problem of the ball-point, though, that I can think of past the finicky soaking (and, as a result, cleaning) issue is that the unit seems to lighten any ink used. As you can see from the scans (bottom of this post), PR Sherwood Green, a usually very dark and assertive green (used in the third scan with the Estie unit), becomes a regular Crayola green with some yellow hue. Hero Washable Blue becomes much lighter, too.
As I mentioned earlier, the "greatness" (is that even a word?) of the pen comes with adding in another company's nib unit to this fun vintage pen (the imprint says Germany W, assumedly before the fall of the Wall). The Esterbrook 1554 Clerical Fine/Med is a fine combination, as the piston-fill gives it a larger ink supply than with any old Estie sac-filler (my previous LJ, for example). I'm also less afraid of staining or otherwise damaging the sac with saturated PR inks, for example, as there's no sac to damage here - though the ink-view window is a candidate for staining, I'm a bit less apprehensive about switching inks frequently with this than with my LJ.
What is there to be said about the nib, though? Pretty much nothing sans the usual comments about the Estie nib absolutely floating through a layer of ink. It really is a great combination, though it sometimes is prone to a slow start. Drawing a circle or two starts it up once again. (I suspect the tiny safety breather hole below the clip - a pinhole size - is to blame.)
The size of the pen is a tiny smaller than I had expected, but does not detract from writing with it. It's roughly the size of a Cross Solo, give or take a centimeter or two for height. The appearance is of a nice burgundy plastic, and the twist cap removal system I prefer versus a snap-off cap. There's gold trim on the cap with a small band, along with the clip (which seems to be able to twist around, if you would like to cover up the little breather hole).
In conclusion, though there are some minor issues with the pen, I believe that it doesn't fully take away from the great price and great base to work from in adding another nibs to convert it to a fountain pen. Can't go wrong with $5 at all (if DWL is still offering them up for that price).