Hi George,
The G6 is superb. I have owned and used one since January.
I even have mine running through a Aquamedic CO2 reactor and Hydor External 300w Heater with not much reduction in flow.
I have not had any problems with it at all. Primes easily, no leaks, quiet.
Good points:
Build quality is spot on
Mechanical filter is a 20sec job to change without opening up the whole filter
Has a chemical chamber as well but I use mine for more bio (again 20sec job to change)
Conductivity sensor gives you a good idea of which way the water quality is going
Digital readout has temp display
The onboard 'computer' can be user set individually for mech, chem and bio media time scales and tells you when they need replacing / cleaning etc
Low flow warning
Supplied plumbing is of very good quality
Looks great (subjective I know but to me has the best 'looks' on the market)
Bad bits:
PRICE
Only holds 4L biological media and does not come with enough media in box. I would say it is very, very optimistic for this filter to suit a 600L aquarium by itself! But IMO that can be said about a lot of filters
Regards
Dave