Flown through T3 many times, including very recently, and never really thought it was that bad. By the same token I would never say it is that great either but I wouldn’t really let it sway my airline choice.
Depends on how long you go back. My first ever flight was into MAN on a BA ATP onto what was the new domestic pier, a haven of calm and professionalism. It's still physically visible as part of T3 but as of last week, the KFC nails what T3 is now. People stuffing their faces with junk on the run or sitting on the floor recharging phones. It's just a very unpleasant environment to be in, only beaten by MAG's destruction of the orignal concept of the once beautiful terminal at STN. BAA used to be known as malls with runways attached, at which point MAG smirked and said "Hold my beer." Please God let the T2 rebuild be something decent...