My experience was that Bluetooth was solid at the end of the dumbphone era (although the UI was often crap). Like you we were bluetoothing files back and forth, I was syncing phone contacts with my Mac over iSync Bluetooth, sending SMS/getting call notifications on my Mac (can't remember the name of that app), etc.

But when iOS/Android smartphones came on the scene it took a huge step back in reliability. A2DP also took a huge step back in audio quality until the new platforms tweaked their SBC parameters and added in MP3/AAC support.