From a passenger point of view, they were a big improvement over the 321's. I used them daily from their introduction for several years. At the time they were probably one of the fastest accelerating EMU's in the country.
Aircon was the biggest improvement (despite being quite noisy) plus a PRM loo. These units were a lot quieter at speed in terms of interior noise, and had a good comfy ride.
Maybe the 'ironing board' style seats in standard was one of the few disappointments. Not everyone enjoyed the warbling sound under acceleration.
Not being a driver, i can't comment on that side of things except to say that those drivers I am in contact with appreciated the superior acceleration and braking of the units compared to the 321's - which was useful when making up time lost due to delays and incidents. They were also one of the most reliable fleets in the country.
Initially, there was talk about them being used on an express Liverpool Street to Ipswich service with only 1 or 2 stops in something like 64 minutes. But they soon started to be integrated alongside the 321's on everyday Ipswich, Clacton, Harwich diagrams.
As a result the 360's performance was never truly exploited as the timetable required some uniformity and needed to be timed for the slowest traction - 321's.
But get on a train delayed by around 5 to 10 mins early on its journey, and these units would literally fly, often making up most if not all of any delay!
Towards the end of their life on GA, the interiors were starting to look very tired and in need of a refurb/refresh. Drivers reported to me that some of the dampers were becoming worn - resulting in a slightly livelier ride and lots more creaks and rattles. It's been suggested this may have been due to a rundown in maintenance as the fleet was being replaced - and Modern Railways MTIN figures reported a reduction in reliability.
The Class 321'R' Renatus conversion was designed to bring those 321's to Class 360 performance levels, as well as the PRM mods and aircon, but I don't know any driver that would take a 321R or standard 321 over a 360. Probably very few would take a 321R over the standard 321's.
As a passenger i would always 'groan' if a 321 turned up instead of the booked or 'hoped for' 360.
From a performance perspective the new 720's are taking over the baton, but again not stretched performance wise by the 321 derived timetable.
They are cleaner/ quieter and more spacious, the aircon is quieter. And drivers are waxing lyrical about the cab environment and performance.
I do miss the 360's but the 720's have raised the bar - even if their fleet reliability is still very low compared to the 360's.