The main issues I have with Flairs is the oven cleaning (the early ones with the chrome liners look great when they're clean but not so when they're not, and they are a pain to keep clean and shiny). If you like to keep your ovens clean, it might be rewarding to keep them clean, otherwise, it can be a pain!
As for the cooktop and surface units, I can't complain, the one I have at the back of my garage where is my tiny kitchen is a real space saver (and that's what I need!). I usually use the cooktop pulled on the first detent so just the two front burners are exposed and it's great. I also like to sit next to it as I can't stand up for a very long time and the lower height of the cooking surface is great for that! I never had a problem using the ovens and cooking surface at the same time but I agree with Tom that bending behind the cooking surface to access the storage cabinet can be complicated if it's fully retracted to expose the 4 burners. Not so bad if it's half-opened!
I think having the vent hood above it is essential, so you want the one that matches it.
As for repair/parts, the first generations are easier to get parts for as they sold in greater numbers and they are also a bit simpler. The 1965 and later models have more complicated oven controls (at least, the more desirable Custom Imperials do). They are often a pain to move and to fix when something goes wrong. I've been waiting for years to fix one of the drawer safety switches that has been bypassed in mine. I have the part but I don't have the space to move around it!
I think these are made for people who take great care of their appliances and enjoy showing them at least as much as using them!
The size of the ovens has not been a problem for me (I can't be considered as a serious cook or even an average one as I don't cook much and this range isn't at home but at the back of my garage in my "play" kitchen!) but I can tell that some large baking sheets just don't fit there.
However, if I remember well, Jon once told me that his Frigidaire radiant wall broiler doesn't fit in his Thermador ovens while it does fit in the larger oven of the 40" Flair (and of course it would fit in the even larger oven of the single oven model).
I have the optional rotisserie attachment in mine but I have to admit I never used it. I'm afraid of the mess it could make and I don't want to deal with the cleaning!