Question: Is a Regenyei Armory feder worth buying?
My verdict: Yes, do it.
Regenyei Armory feders are ubiquitous in HEMA clubs across Europe and North America, and they are popular for a reason. They tend to be well-made and they tend to last for at least a couple of years, while not being particularly expensive for such a good training tool.
They have a very good reputation, and they also come with a warranty for breakage from normal use within a certain timeframe, so you can have peace of mind that they are well-constructed (if not, you will receive a replacement) and that the smith will take pains to make sure that it is sufficiently well-made that they will not need to replace it.
If you have the opportunity to get a Regenyei feder, it will be worthwhile. Ideally, choose one of the medium blades (the light blades are too flexible, the heavy blades are too stiff to be safe), so that your training partners will be happy to let you practise with them.
Keith Farrell teaches HEMA professionally, often at international events (why not hire me to teach at your event?), and has an interest in coaching instructors to become better teachers.