It's one of those things that people want, but never get into for the most part. Midi-enabled guitars have been around since the 80's, and though they have gotten better by leaps and bounds, they have never really caught on. I personally dove headfirst into it more than 5 years ago. I got a Roland GR-20 with a GK-3 pickup.
So here's a tutorial if you've got a midi guitar laying around and want to give it a whirl.
I went for the cheaper Roland unit available at the time [the gr-20] as already softsynths were becoming popular - I figured in the future that would be the way to go, and don't plan on busting out the g-synth live anyway. It still kicks some butt.
As far as tracking as a midi instrument, your technique has to be pretty good - any string squeels or muted notes will show up in your performance. You have to work it like a machine! If your technique isn't up to the task, you will spend more time editing your performance than you spent laying it down.
So my advice is to track using a percussive sound, with lots of attack - for example, piano or my favourite as of late is bells - you can really hear if you are in time when you hear the transient. Playing with big pad sounds is fun but harder to check your timing with. You can always try out the sounds after you've recorded the part.
So let's say you recorded your synth part. The next step is to put it in time - a midi guitar needs a split second to analazye the notes before it can spit out the midi. It usually show up on the beat; I just take my entire performance and slide it so that the first note lands at the correct place in time. If the rest of the performance was good, then it should all be in time now. If you're going to quantize, make sure you compensate for the delay caused by your midi unit first, or else there'll be hell to pay.
Next what I do is open my velocity view and highlight anything hit really lightly - generally what this will do is select all of the ghost notes that showed up - muted strings, squeeks, etc. This generally leaves me with a clear performance, and I'm ready to dig into my sound banks to choose my tones.
The hardest part of tracking midi guitar is losing all of the mannerisms of a guitar player - for example, not adding vibrato where I normally would if I were playing "guitar" is hard.
One cool thing that I really enjoy about my midi guitar is that every string can send it's own midi channel, so I am almost always coming up with "super patches" on my soundbank. My latest favourite is an orchestra - I'll break it down so you can imgaine how well this works.
All strings play a violin patch - my synth knows the ranges or the violin family, so lower notes are played back as a cello, mid notes are viola, and high notes are violin.
The bottom 3 strings play trombones and tubas
The top 4 strings play harp
The bottom 2 strings play bass
When I play some sweet fingerpicking stuff, it really comes alive. Playing with the attacks really makes it suit the different tempos - for example, if I put a slow attack on the horns, they will only come in if I hold a note etc etc.