Conclusion: The TWSBI Eco, Eco short for Economical, is a value for money piston filler fountain pen. If you are looking to dip your toes into the world of piston filling fountain pens, the TWSBI Eco is a great place to start. I wouldn’t recommend the TWSBI Eco to be a starter fountain pen. It might be a little overwhelming for a beginner to learn how to service a piston filler if they came from the world of disposable ballpoint pens.
The TWSBI Eco is very interesting product. I'm not sure if it is a hit or a miss. I wouldn't recommend it as a beginners pen. I think the Pilot Metropolitan is a very good beginners pen because it is cheap and you just pop in cartridges and write.
You have a good pen you paid around $15 for, and you like fountain pens, and want to test out some more inks. You could either buy a converter for less than $2 or find another pen. You could either buy a pen in the same price range, or save a little bit more and get a much better pen for $60 dollars instead of the $29 the Eco costs.
The reason I'm not sure if it is a hit or a miss is that I don't think a piston filler is for the beginner market. So the only places I can imagine that this pen has a market is for people on a tight budget, or if you want to just have some good and cheap pens. For example laying around at work where it wouldn't be too bad if a few of them disappeared.