GibsonLesPaul,
I have found that music mixed and sold as multi-channel will give the best results. In my experience the best formats are DVD-A, DVD-V, SACD, DTS-CD, and open reel. All those formats are capable of providing multi-channel music with discrete information in all channels.
In the "Quadraphonic" era there were several matrix formats that in my humble opinion, never worked very well. One could play back a two channel stereo music signal through one of the matrix decoders and get a "pseudo-quadraphonic" effect. Sometimes the results were even better than what one got with recordings made specifically for quadraphonic playback. Among the better decoder/synthesizers of the quadraphonic era were the QS decoders found in the QRX series of Sansui receivers and the Tate SQ decoders. Today one can buy receivers with Dolby Surround IIx, DTS Nero 6, and Circle Surround decoders/synthesizers that can rival the best decoders of the quadraphonic era.
The short answer to your question is that with one of the better decoders of the quadraphonic era or one of the modern decoders one can get very respectable results synthesizing four channels from two channel recordings. However, the results vary based on the specific recordings fed into the decoders. Some recordings will sound much better than when played back in two channel and some will sound worse. The results will also vary with each of the different decoders. I find that I prefer to listen to some recordings in two channel and others played through the Circle Surround decoder built into my receiver.
I hope this is helpful.
Good listening,
tcdriver