Is Cangjie (or Wubi) worth learning?

YES, it is! And that's been my experience so far.

Why?

1. I have been learning Chinese for the past few months, and Cangjie for a little more than six weeks. I don't know many characters, my grammar is poor but when I see an unknown character online, I can look it up in a dictionary without having to copy it which is anyways impossible when the unfamiliar character is part of an image or shows up in video. So that's an advantage of learning Cangjie (or Wubi): you can type unknown characters.

2. You can't use Cangjie (or Wubi) if you don't know what 漢語 looks like. So the system compels you to remember the shapes of characters. This gives you extra practice when you are learning Chinese. With Pinyin-IME (or Bopomofo), you tend to forget the details and after some time only a vague form remains in your mind. At least that was happening to me.

3. They say Cangjie (or Wubi) is faster. I confess I still type faster in Pinyin. But then I have been used to Qwerty layout for the past so many years. I believe after a few months, I will be able to type faster using Cangjie.

But they say...

They say it takes a long time to learn Cangjie (or Wubi). 

Switch to Dvorak and I bet you can't match your current typing speed on the new layout for at least a month. And that's normal. It takes time to learn anything new. Cangjie is no exception. You will have to spend a month, or in a few cases two, before you can grasp its principles and feel comfortable with the new layout.

They say you have to memorise a lot of codes.

This shows ignorance on part of those who makes such ludicrous claims. It's just not true. Do you think I learnt to type 漢語 by learning this code etli yrmmr? Of course, not! That's impossible! ? You only have to learn 24 + 1 letters and their auxiliary shapes. Together they don't amount to more than 100. This may sound a lot, but at the rate of ten-a-day you can have them all under your belt in less than ten days.

There doesn't exist a tutorial in English.

That's partly true. If you are ready to spend a few rupees, there does exit a book in English to teach Cangjie. I am not sure about Wubi. If for any reason you don't want the book, these links may be helpful: 

1. Introduction to Cangjie

2. How to learn Cangjie

3. Chinese-English Dictionary with Cangjie Codes

4. A pictorial dictionary that demonstrates how radicals and sub-radicals come together to form a character

In case you are learning Chinese through simplified characters, these links may prove useful:

1. Introduction to Wubixing

2. Chinese-Chinese Dictionary with Wubi Codes

3. A pictorial dictionary that helps you understand how the method works

Are there any downsides? 

One of the downsides that I have noticed so far is that I sometimes forget how to pronounce characters, even though I remember how to write them and what they mean. Today I learnt the word for bicycle. It's 自行車. I have forgotten how to pronounce the first character but I remember the remaining two are 行 (xing) and 車 (che).

Do people use them?

Cangjie and Wubi are not as popular as Pinyin-IMEs. Perhaps there are ten (or more) Pinyin-IME users for each Cangjie or Wubi user. But then Macs aren't as popular as PCs, and there aren't as many Ferraris sold here as Maruti Suzuki Altos.

How to install Cangjie or Wubi?

Linux (Fedora)

Go to Add/Remove Software, type Cangjie (or Wubi). There will be several options. I use Smart Cangjie. For Wubi, you can choose Wubi-Jidian86. After that click on Apply. The system will ask for Root password, type in that and you are done.

Windows and Mac

I don't know. It's been quite long since I last used Windows and I have never used a Mac.