For an instrumental piece to be a good candidate for recognition it helps to have as many of the following properties as possible:

• A low degree of polyphony.
  Stick to trying to recognise just a few notes at once.
• A strong fundamental frequency and weak harmonics.
  Wave Goodbye does not (yet) support harmonic recognition. If the harmonics of a note are too strong then they may be recognised as additional notes.
• The fundamental frequency of each note must be stable.
  If the detected frequency varies too much then it will trigger recognition of the surrounding notes as well.

Piano music is about the best possible in these circumstances.

Anything which breaks the above rules will suffer in recognition accuracy. A highly polyphonic piece can still be recognised fairly well, but setting the polyphony control to low will leave gaps and setting it too high will introduce incorrect notes from any strong harmonic presences.
Organ music and a lot of synthesised sounds contain very strong harmonics which will lead to the false recognition of additional notes above the fundamental frequencies.
Any instrument with non-discrete tuning (i.e. which can produce any frequency within a range rather than being constrained by its design to certain steps) such as string instruments can often be played with enough variation or vibrato to blur what should be a single note across the detection boundaries to its neighbours. Stringed instruments also generate a lot of strong harmonics.
The worst possible sound to recognise is the human voice. It is unstable and contains too many harmonics. The only thing worse than a solo singer is a choir.

On a 500Mhz system conversion is done in about real time.

The only constraint is available memory and swap space. Unlike the demo versions of commercial wave-to-midi packages there are no artificially imposed limits.

There are many commercial, shareware and free midi file editors available. The internal version is intended for very minor changes only and is unlikely to be improved in the near future.