HellbentMYLESPugs can teach you what other classes do, positioning, and even strats.
I review invite/IM demos usually as a whole, like an overhead view to get an idea on the whole game rather than just one person.
fuck this turned into a rant, sry
This is an important thing many players don't realize. When you watch overhead you get a much better idea of how a team, or even an individual plays. First person speccing may be more entertaining but you're blind to more or less everything, seeing how players interact and position relative to each other will teach you more about the game than watching some dudes mouse movements when you have no idea what info he has available from comms.
Even if you just wanna watch one person, the overhead will give you a much better idea of their awareness and ability to react to whats happening around them than pov
if you watch the pov of a REALLY good player(especially great demos + meds), it's a different experience than most povs. When watching banny for example, he keeps basically everything important on his screen at all times. It's partially positioning, and great awareness + comms in mumble, but he knows what is happening everywhere, at all times. It's almost impossible to catch him off guard because he keeps his eyes on every path that someone could be using, or he KNOWS his teammate has eyes on it and can be relied on to give the early call. Knowing what exactly is going on around you via calls is great, but you will never be able to react as quick as you would seeing it yourself.
Being caught off guard is your own fault 90% of the time, sure sometimes you won't get the call etc etc, but most of the times if you had been in a better spot, turned a different direction so you can see choke + flank instead of just one, you won't get surprised by the scout who's been hiding for 30 seconds. It's just like an rts, if your mechanics are solid and you know how to handle most situations, the player who wins is the one who has the most information about what his enemy is doing/his teammates are doing