People buy into systems for different reasons. If you want the latest, secure and largest network around, the Hy6 and DHW won't fit the bill. They are a very small company making a very fine product, mostly in continuation of a design that was superb when introduced and is still one of the most flexible and finest pieces of gear out there.
There is disagreement as to how active a manufacturer DHW is, with opinions varying from "assembling old stock" to actively making new Hy6s. Its not clear who owns the intellectual property rights to the Hy6, and whether anyone will pick up the system moving forward as a platform.
DHW is responsive (in an old world way) to providing rapid service. They provide both limited international distribution and direct sales. I've dealt with them for years and have only good results.
Neither the Phase IQ or Leaf Credo backs currently mount to the Hy6, but the Leaf AFI II 10 and 12 do and are still made. Older Leaf backs that were made for the Hy6 (or AFI) are another option, but hard to find. Sinar backs can still come with an adapter to fit the Hy6.
The sizes of the AFI 12 is comparable to the IQ180, but the newer backs are more up to date in software and processing speed. That said, the AFI backs have a rotating sensor, which is just a delight to use. I have an AFI II 7 back with this proviso and can't imagine working without it.
The Hy6 has an extremely broad base of accessories, and can easily use the older PQ lenses, both manual and AF. Focus confirmation is very good, making use of the less expensive manual glass a real option, one which many of us have heavily supported. Schneider glass from 1990 on (if not earlier) is all usable and very fine. Edge to edge sharpness at 100% on all the lenses is typical. Some think the Schneider lineup for the Rollei (and the Hy6) is one of the best examples of a lens lineup. There are many special and lovely lenses available.
The Hy6 is one of the first "intelligent" MFD cameras, with adjustable focus offset (you can set for each lens as needed, very useful for sharp tuning), histogram on the handle for easy reading, changeable finders, backs, etc. Newer cameras are even better; however the Hy6 is quite usable and has excellent characteristics, both electronically and in the hand. Distribution and availability isn't ideal; the users are supportive of each other and very fond of their gear.