I got one of those 'modern' 24/2.8 FD's for use on a T90. It's a decent lens alright. Not stellar, but I've seen (and used) much worse. Sharpness is...okay. Barrel distortion is for the most part surprisingly limited given the small size of the lens. Flare behavior is indeed quite good for a lens this age - not so stellar compared to modern lenses with contemporary coating technology. As with all wide angle lenses, diffraction becomes a real problem at f/11 and beyond unless you print small - which, fortunately, I do most of the time, so it doesn't bother me too much. Conversely, wide open, it can be a little soft and the corners never become quite sharp, but for a wide angle lens this size, it would be unrealistic to expect any different.
Overall it's a pretty useful compromise between weight/volume and image quality. Overall I enjoy using it - and perhaps should use it more often than I do!