Transcription (learning tunes/parts/solos by ear) is one of the best methods for improving as a musician. I mean, what could be better than learning directly from the source?!
There are countless articles online about transcription, but let's take a slightly different angle. Let's look at transcription from the point of view of the recording guitarist.
If you've done any recording at all, you're well aware of the fact that it's a totally different animal than live playing. Here are some ways to improve your recording chops via transcription. Learning the notes/rhythms is a given...these tips will take you deeper into the mind of the recorded guitarst and even the producer and mix engineer.
1. Match as closely as possible the guitar tones from the recording. First, try to figure out the guitar (or type of guitar) you're hearing. Humbuckers? Single coils? P-90s? Jazz box? Strat? Tele? Les Paul? Next, see if you can figure out what's going on with the amp. For starters, is it a clean or dirty amp tone? As your ear develops, try to recognize the sound of specific amps...Fender, Vox, Mesa, Marshall, etc. Lastly, listen for the effects used. One word of caution here...when recording with effects, a little bit goes a long way. When you're matching the tone of the recording, be careful not to overdo it (in terms of effects) and end up with a caricature of the original tone.
2. Listen to how the guitar tracks are layered and panned. Where does each part sit in the mix? Are there doubled rhythm parts panned hard left and right? Does the guitar double another instrument? How do the guitar parts fit with each other and the other instruments rhythmically?
3. What role does each track play in the tune? A well-produced song has exactly the tracks that are needed...no more, no less. Therefore, every guitar part is there for a specific purpose. Try to get into the mind of the producer and discover the role each part plays. Is the track a featured guitar hook? A texture part that glues the tune together? A support track for the vocal melody? A double of the bass line? Melodic fills that keep the tune moving forward?
4. Match the feel of each track. "Feel" refers to musical concepts like pick attack, articulation, dynamics, slurs, vibrato, etc. These are the subtleties that bring a guitar part to life. I'd go so far as to say that "feel" is one of the biggest separators between pro session players and amateurs. Work to pinpoint these inflections and bring them out in your own playing.
One great way to put all this info to use is to take a song you like and transcribe ALL the guitar parts. Record every guitar part you hear, and make it resemble the original as closely as possible.
This kind of practice will train your ear (obviously), plus it will force you to really learn your gear. On top of that, it will give you a glimpse into the mind of your favorite session guitarists and producers!