I haven't flown a 400 but spent many happy hours in a 260C, more or less straight after my PPL. It was quite a lot of aircraft for someone trained on a C150 but I had very thorough instructor and came to the PA24 without having heard the many old wives' tales about its operation. I found that it was a great aircraft, able to be flown by the book. Thirty years later I still rate it as one of the best aircraft I've flown.
In terms of conversion experience, the speed was quite an eye-opener but needing to stay ahead of the aircraft is common to all more advanced types.
I'm not sure the 400 represents any sort of sweet spot in aviation (lot of l/hr) but if you have a particular motivation or curiousity, it'd certainly be fun trying it out.