You've got to view this as a reflection of the Sexual Revolution in its full 70s "swinging" mode, where infidelity within an unhappy marriage was viewed as less simply immoral than now. As such it's a charming time capsule with very good casting. I thought this movie was terrific (albeit imperfect) at the time, when I saw it as a teenager. Of course it seemed very sophisticated to me then--and it made me infatuated with George Segal, who seemed so goofy and charming and attractive. A perfect post-Bobby Sherman interest for a curious 12- year-old.
As for Jackson's Best Actress win--well, there really weren't a lot of good roles for women at the time. This is a particular instance (like Louise Fletcher's very-supporting Best Actress win for "Cuckoo's Nest") that proves how dismal the competition was in that era. Of course there were fine actress performances in films during the 70s, albeit ones too small or too foreign or too art-house-y to be noticed by the Academy. But really, the whole era just sucked in terms of substantial women's leading roles.
The film's own dated sexism is apparent in its obliviousness toward divorcée Jackson's drop- everything-whenever-called neglect of her children (guess she has nothing better to do!) whereas much attention is given to Segal's neglect of his wife and children (he's a guy, so of course he's got better things to do!). As if her commitments aren't important, while his naturally are.
"A Touch of Class" seemed overrated at the time (this movie got multiple Oscar nominations in the same year as "Mean Streets"?!?), and it hasn't aged brilliantly. Nonetheless, it's an excellent example of a romantic comedy reflecting a very different moral complexity than movies allow nowadays.
