Answers to questions, some not asked.

Called ID on the 848 has a quirk; it does not log globally on answered calls.
Programming is different then the 624. If your distributor does not provide tech support, plan on visiting with the pleasent folks in Jersey.
Executive override is different for this system then with the 624; it requires an extra button push to bring somebody on a call
Ring groups are broken into smaller clumps.
Cordless performance is demonstrably better then previous generations, and I would bet, better then Avaya
Panasonic sponsered training is starting to appear at select distributors around the country. I know of one in June that lasts two days, will offer hands on training, and will be limited in size to 12 participants.