I was drawn to FRA because of the job opportunities and lived there for a short while but hated every minute of it. To me it seemed like there was no beating heart/life to the city. Unless of course you're into suit guys and corporate c..., then I'd imagine you'd be in hog's heaven.
I didn't find the FRA locals that friendly, the Amt on Mainzer Landstr. was awful and they took ages to process something. The bars were half empty - even on a weekend, and I found it really difficult to strike up conversations as it was terribly "
cliquey". Ironically, all the down-to-earth / up for the craic people were all working with the Airlines - I still travel there to see them every now and again.
At the time, it seemed like FRA was a place where everybody works and nobody lives. My German friends always refer to it as a BWL stadt. I haven't lived there for a long time, but I do hope it has changed since then.
As for Berlin - still not quite a capital city like Paris or London etc... but it definately has a certain something about it. I know a few americans there, and they always seem to say that the Berlin spirit/attitude is similar to NYC - I guess that's open to interpretation...