Cleaning your Esterbrook
Thanks to the removable nib units, Esterbrooks are easier to clean than many other sac-filling pens. If you're a patient person, you can submerge the nib in water and pump the lever gently until the water comes out clean. If you're more thorough or less patient, you can remove the nib unit and rinse out the inside of the sac directly. You might try using a blunt syringe to shoot water into the sac, but, of course, don't poke the sac.
About Those Nibs
You know how you might go into a coffee shop and order something like a full-fat, half-caf, hazelnut latte with an extra shot of vanilla? (Not my thing, but I've seen people do this.) Customization is fun! You're not just ordering a generic cup of Joe--you're ordering this highly specialized drink that you experience as uniquely yours.
Esterbrook figured out this customization thing long before Starbucks. Their interchangeable "Re-New Point" nibs come in dozens of varieties, tailored to specific purposes. They even offered nibs specifically for use with the Gregg shorthand system of writing. The nibs are all steel, but for the most part are of high quality. I've heard of duds, but since new ones are so readily available, even if you have the bad luck of getting an Esty with a bum nib, you can replace it fairly cheaply.