Nothing like opening the proverbial can of worms Oso. Immediate disclaimer, Esterbrooks can be habbit forming.

The appeal of Esterbrook is many. Like you've said, they are durable pens. The plastic is fairly resistant to warping like other third tier or lower pens and discoloration is almost non-existant, unlike higher priced Parkers and Sheaffers. The plastic is generally available in black and five very pretty marbled colors, so there's a color to suit just about anybody's tast. Also, since the plastics are fairly durable, you'll rarely find one with a crack in it. Mind you, it does happen, but I've handles hundreds of them with only a few cracks seen in the last seven or eight years.
Another appealing element (at least to me) is the lack of gold plated trim or nibs. There's nothing I hate more than to see a pretty 1930's pen in a beautiful plastic, but the cheap gold plated trim or nib is worn down and it looks ugly. Not so with esterbrook. About the worst you'll get are bandless dollar pens which used a different mixture of metals for their clips and levers and they often rust. The rest are practically like they left the factory.
The interchangeable nib was not Esterbrook's idea, but they really stuck with it and ran. Since renew-points (or re-new-points as they were earlier called) are interchangeable amongst all models, there's practically an endless supply of them available. Got a bad nib? swap it out! Many are available NOS (New Old Stock) in their original boxes. They great thing is if you find a nib you really like, you can get a bunch of holders (pens) and swap the nib amongst them. Feeling like a Blue pen? swap it out. Want a Copper colored pen to match that copper shirt? No problem. Want to put it in a later cartridge fill or aerometric fill pen? Again, not a problem.
There are really two models I would call "quintessential". The double jewel J pen and the model that proceeded it the flat top model B, or more commonly referred to as the "Dollar" pen. Both series came in full sized (not Oversized as some ebay auctions would like you to believe), slender, and a short (or demi or ladies sized however you want to call it) sized pen.
Ebay can be a (Potty Mouth) shoot, but if you know how to replace a sac you shouldn't have much of a problem. Things to look for aren't unlike any other brand, but the biggest problem you'll probably run into is a bad or scratchy nib. Buy yourself a could NOS nibs and you'll be in business. The only other problem you might run into is a sprung clip. Since the clips aren't washer style like a Parker, they aren't easy to bend back. You'll know if a clip has been rebent incorrectly as there'll be a hump in the middle of the clip.
If you don't know how to replace a sac, there's no better pen to learn on and many here can help you through. You'll find Esterbrook was the "learning" pen for many people when going down the road of pen repair.
If you want to avoid ebay, many folks here at FPN wouild be happy to sell you restored Esterbrook pens. I've been known to have one or two of them for sale myself.

Contact me backchannel if interested.
Oh, and of course, Welcome!
Best-
Brian