The main problem is that the mechanical signal of a vibrating string produces an analog electrical signal than conveys pitch, volume and dynamics information. Converting the analog to digital has become routine with today's computers, but going one step further to convert a digital signal to Midi note and controller information is more difficult.
Midi conversion has to determine which note is to be played from the frequency content of your guitar. If your guitar is not exactly in tune (are they ever?) then the coverter must make a guess at which note is intended. If you bend a note or add vibrato, the converter must add the appropriate pitch-bend controller information. The converter must also determine volume and dynamics from the signal. Volume is probably easier than dynamics. Did you pick hard or soft, do you want the signal to decay like the normal guitar signal? Then you have to multiply the problem by six (a signal for each string).
The older midi converters did not have enough processing power to provide all of these conversions in real time, so you often had to contend with delays and tracking (wrong note) problems.
The good news is that the current generation of midi converters are much more powerful and robust. This is a direct result of more powerful processors. Roland pickups use a 12-pin cable to the converter but the processors have standard 5-pin midi connections so that you can control other midi devices, such as keyboards and sound modules. Shadow makes a midi converter that provides 5-pin midi to control other devices without the processor that Roland uses. I think that the Shadow costs around $750 US. Several guitars are now available with either direct midi control or Roland-compatible pickups (Brian Moore, Godin, even Fender).
Another thing to keep in mind; guitar midi converters go obsolete very quickly due to fast-changing technology. I think that the technology is fast enough now that I might risk buying a Roland VG-88 or GR-33.
