I haven't but people seem to be pleased with their purchases from them. I tend to use e-infinity uk when I am purchasing new gear.
Checked out e-infinity a little and found: prices are similar (only a new Z8 costs $3400 at e-infinity as the lowest from everywhere, and costs $3553 at Panamos, $4000 at Nikon; other cameras cost about the same); e-infinity offers fewer products; e-infinity provides a one-year warranty (assume same local or non-Nikon service as Panamoz), Panamoz provides 3 year warranty. For US delivery, e-infinity ships from Hong Kong and Panamoz from the US warehouse. Delivery time is similar ~ 7 days (wonder if from Hong Kong could take longer). Nothing much can be found about the e-infinity company. I doubt products from either seller are allowed to be registered at Nikon USA and receive paid services from Nikon USA (but some people did).
You are correct to assume that you will not be able to register a product purchased from these companies with Nikon. Well that is certainly the case in the UK.
For the products you purchased from e-infinity, did you ever need services or repairs? From their service shops? I asked Panamoz about my area's local service shops and they said any local service or repair shops will do (they'll accept the bills). That sounds like promising. I may take a plunge...
I never required any repairs from any products that I bought from them. I think the chances of you requiring any repairs in the first few years of ownership must be extremely low. These new products will just be like any other new products from other dealers, albeit without the excessive tax charges added.