So why would an aspiring guitarist want to learn how to play Alex Lifeson's guitar parts to Rush songs?
Well, there's a lot he can teach you. I can summarize with two words: "fast" and "loose."
He's very fast. He can shred with some of the best of them, playing flurries of notes. But unlike some guitarists, he rarely sticks to a straightforward pentatonic or blues scale. That's valuable to learn. Beginners have a tendency to stick with the scales they know.
He's also a very loose guitarist -- he can zoom up and down the neck with abandon. He very rarely plays a straightforward rhythm part in a single locked-down position; instead, he's always playing very wide chords, blending open strings with fingerings halfway up the neck, and adding individual notes to arpeggios or chords.
He often plays partial chords, skipping strings and/or muting strings. These can be hard for beginners to master, because we're initially taught to strum all the strings, and tend to default to that. Learning how to play only partial chords accurately requires precise right-hand technique.
He often plays runs of notes "horizontally," along the string, while beginners tend learn to hold a fixed position and play in scales. Playing his lead lines will force you to get your hands moving along the neck. For example, the solo in YYZ comes in four parts; the last of these four parts is a flurry of picked notes, followed by a melodic series of hammer-on/pull-off notes, played entirely on one string.
Often his solos that seem to contain a lot of notes actually only contain a few -- he repeats notes, something beginners tend not to do, as they solo up and then back down a scale.
He uses a lot of bends -- string bends with the fingers, single and double bends, and bends with a whammy bar (or "tremolo," although this is a misnomer). I've never been able to get the hang of a whammy bar -- in part because the guitars I had all had really poor whammy systems that would throw the instrument completely out of tune if you touched it. So aggressive and precise use of the whammy bar is something I have yet to master.
But there's a downside to all this: it can be easy to get partway there, to play something that sounds vaguely like a Lifeson guitar part. But it is very, very hard to get the rest of the way there.
Because his parts are so unusual, most transcribers tend to get them correct only to a certain point and then punt, writing down a few chords. Even the tab you see that ought to give detailed fingerings is often wrong. Even when they are right, they may seem impossibly difficult to play. That can be discouraging. To the best of my knowledge, Lifeson has not released a "how to play like me" video training series. It's a bit of a pity -- it would probably sell quite well, since he is really a musician's musician and there are a lot of aspiring guitarists who would like to know just how he does what he does.
In their studio recordings, such as 2112, Lifeson often layers multiple tracks, using three or more guitar lines. It can be a bit of a challenge to pick them out, so there's an honest tendency to take the average -- "oh, he's riffing on an A major chord" -- and just play that. And it sounds mostly right, although if you go that route you haven't learned very much about what Lifeson is actually playing.
Is it any easier to learn off a live album? Well, this can be a little easier, but when playing live, Lifeson often seems to use even more effects -- more flange, more distortion, and more reverb. The distortion together with his artificial harmonics and flange tend to turn one guitar into a wall of sound. This makes it hard to pick out the details. A river of thirty-second notes can be pretty intimidating as well, especially in a Spanish Phrygian scale. He also works in feedback from his amplifiers, which might be hard to copy, especially if you're trying to play in an apartment with noise-sensitive neighbors.
The sheet music is spotty. Most of the Rush sheet music that is available is really arranged for keyboard, although sometimes you get chord symbols. There are some tab transcriptions. Some of them are quite inaccurate, but some of them are pretty good. So far, the best stuff I've seen is in this book, which features detailed tabs, although even these still seem to have some inaccuracies. The tabs available online tend to vary a lot in quality. Even if you have a very good tab, you may still have the challenge of figuring out exactly how to finger the part in question. That often comes down to a matter of taste and a guitarist's individual hands. There are some reaches and fingerings that some guitarists can play that I can't, because my hands are smaller.
But don't give up! You can learn a lot, if you are willing to put in some hard work.