It’s Dominic (no K at the end), named after Dominic O’Brien, eight times world memory champion. So, that tells you how effective it can be!
However, it’s really not possible to answer your question without knowing what you want to memorise with it. Also, how do we measure effectiveness?
Easy to learn?
Ease of use?
Expandable?
Ability to memorise items where speed is the issue?
Ability to memorise items where amount is the issue?
Suitable for long-term memorisation?
And so on.
You can think of both Dominic and the Major System as number-letter codes. They are effectively ways to encode numbers into letters and then into images.
Dominic would be:
1 A
2 B
3 C
4 D
5 E
6 S
7 G
8 H
9 N
O O
The double digits then (usually) become initials of a person. E.g 56 = Ebeneezer Scrooge.
Major lettering is different:
0 s, z
1 t, d, th
2 n
3 m
4 r
5 l
6 j, ch, sh
7 c, k, g, q, ck
8 v, f, ph
9 p, b
Consonants are then used to fill out words. E.g 56 = Loch or lush.
Before Dominic, those words were mostly objects, though some people now use Major for people.
Here is how I personally would rate the questions above:
Easy to learn? - Dominic
Ease of use? - Dominic
Expandable - Major (Dominic can grow into a PAO system, but I still think Major wins here.)
Ability to memorise items where speed is the issue? - Debateable, but more top athletes seem to use Major (or one of its variations like the Ben or Shadow systems), than Dominic
Ability to memorise items where amount is the issue? - as above, if we’re just talking e,g. 5000 digits of Pi. If we mean something like 50,000 decimal places of Pi, or 52 packs of cards, see below…
Suitable for long-term memorisation? - Draw
Regarding books, I’d recommend getting Dominic’s first book, How to Develop a Perfect Memory. It’s an exceptional book and most of his later books are just rehashes.