Had a G602 for many years, and then migrated to M720 triathlon. I like the G series ergonomics, but the G604 kept breaking on me. M720 has treated me well so far and support for standard battery cells is very nice.
What broke? You can replace the switches, stock omrons are pretty garbage and it's super common for them to break.
I've replaced the switches in my G604 once, but I don't think I'd do it again. The internal construction is overly complex, so the process took far too long — even though the actual desoldering and soldering was easy. I just wish Logitech would re-release this model with decent silent switches and without the rubber coating.
Just want to plug the M720 some more. That mouse kept me going through years of work/personal computer use.