The Leslie 122 is a speaker cabinet that is to be used with an organ. Specifically, I've used the Leslie 122 rotating speaker cabinet with a Hammond M3 organ. There aren't very many characteristics to the 122 that I know about, as it is a pretty straight forward Leslie speaker cabinet. You can only connect it to a organ, as it doesn't have a preamp to support anything else, although you can certainly get it modded to use it with an electric guitar...
You don't need to do anything except hook up the Leslie 122 to your organ of choice to use it, as there isn't anything you can do to change the sound or anything. All of that is done within your organ, including turning on and off the rotating speaker. I don't know anything about the manual for the 122 if there is one, but it certainly isn't necessary unless you're looking for a schematic or something like that.
The Leslie 122 is up there with the best sounding Leslie cabinets for organ. While it's hard to distinguish the sound of the organ and the sound of the speaker cabinet, without a doubt you'll need both to get a top quality sound. Having only used the 122 with a Hammond M3 organ, it's hard for me to say how this compares to other Leslie cabinets like the 127. I do know however, that the combination of the M3 and the Leslie 122 is as good sounding of an organ combo as I've used. The Leslie 122 certainly does it's job with the rotating speaker, and is widely recognized as one of the best Leslie speaker cabinets out there.
I can't say that I've used a better Leslie speaker cabinet than the 122, as it's pretty much the perfect Leslie cab. I don't know if there are any major differences between the 122 and other popular Leslie models, but the 122 seems to be as complete of a Leslie as you can get. I do wish that I could try it with another Hammond organ to get a comparison with the M3, but either way it's clear that this is as good of a Leslie cabinet as there is.
There isn't much really that hasn't already been said about Leslie speakers. An invention of the late 20th century, and featured on countless recordings between the 60s and today, it's a staple of recording in general, let alone organ performances. It's actually inspired countless effects pedals and other simulations. The basic premise is that it is a rotary speaker, one that spins in a cylindrical fashion while running. This produces a really cool, natural sonic effect that is rather cool. It's basically a natural version of the effect that is often referred to as tremolo or sometimes vibrato. Half of the legend of the Hammond organ is a result of the Leslie Speaker, without it, the organ loses much of its character and really doesn't work as well for most sounds.
The debate as to whether to buy a Leslie is different nowadays. Basically, if you own the organ, you need it. It's worthless without one, and getting a good one is important. However, if you are going to make like George Martin or Tom Dowd and pick it apart to hook it up to other instruments, you might consider a vibrato/tremolo effect that mimics it, simply because of the price and labor. It's a very unpractical endeavor with a rather limited payoff, unless of course you are trying to record a big-budget record. Most people wouldn't even consider it. I'll tell you though, it will be hard to find an effect that can truly match the speaker, especially in the studio. So basically, be wise when purchasing, but if you are going to get a Hammond organ (especially a B3), the Leslie Speaker is essential.