ABRSM is either too overrated or too underrated. I know it very well and I even have some books. It's not bad, it does what it does: it systematically organizes professional musical knowledge, but very basic, yet serious; on the website you can download the 'syllabuses' for each level with the required pieces, etc., for free.
Furthermore, the numerous 'books' are too many; you need one on scales, one on arpeggios, one on this, one on that, and for every grade. Each 'book' has about 20 pages. But complaining about these points makes little sense: that is the system, take it or leave it.
As an experienced self-taught musician, I can say that it is better to follow ABRSM than to have no system at all. In fact, I started to really work hard on my practice a few years ago, precisely upon becoming aware of ABRSM.
Anyway, criticizing it makes no sense; it's like criticizing the invention of the bus ticket. If the bus works for you, feel free to use it, otherwise choose another means of transport; certainly, ABRSM takes nothing away from anyone, and from what I understand, the proceeds also go to charity, so I believe the whining is useless.
There are similar systems to ABRSM, such as LCM or the Trinity College system, anyway. In terms of quality, some say that ABRSM is 'better' or 'more recognized', but in my opinion, that's bullshit. Some ABRSM programs only go up to level 5, like in the Jazz piano program, which in the LCM system reaches the highest level, level 8; therefore, there are things that ABRSM does not offer at all compared to LCM.
Other differences are, for example: in ABRSM they will ask you to perform many scales, while in LCM or Trinity (I don't remember which of the two) you can choose to perform studies recognized by the systems instead of scales.
The problem is this: believing that ABRSM or others are a complete musical preparation system for those who make music their life: they are not at all, and it is only what I said before, a BASIC professional preparation. A starting point for those who are serious about music, nothing more. But in my opinion, it is something, and furthermore, they are well-organized systems; you can even find exams (in English) on YouTube.
http://www.youtube.com/watch?v=plA1CQDnins&feature=plcp
Then it is also ridiculous to demand from the system itself, for example, to be able to choose, say, among Beethoven sonatas to pass a level 8 exam. There are people who prepare for their exam for one or two years, and then end up with all they can do being playing a Beethoven sonata. But it is certainly not the fault of the system itself. A musician who calls themselves an aspiring professional is not a child, and no one better than themselves can decide which improvements to make or which paths to change.
Anyway, in my view, they are all valid systems; ABRSM is no joke—if you don't know how to perform at least well enough all the scales they ask for, sight-reading, ear tests, pieces, etc., you won't pass. In my opinion, there is no sense in either overvaluing it or undervaluing it, and it is anyway better than if it didn't exist; it's not like they are taking anything away from anyone. It is a series of exams that requires basic preparation but is carefully recognized by the system, more or less. Personally, I have nothing against it; on the contrary, in my opinion, it's good that it exists and as a musician I approve of it very much; if someone doesn't like it, they are not obliged to do anything.