Are they worth the money? Are they useful? What are the benefits? I'm going to answer these questions from my experience as a practicing Musician & Guitarist. Of course it all depends on the individual and what their aims are with their instrument, so before you make any decision about how you should be spending your practice time and your money, you should figure out these things:
What are my short term goals?
What are my long term goals?
What kind of styles do you enjoy most?
Firstly let's get the pro's and con's for getting the actual qualification out of the way, there are a number of boards and each of them is supposed to carry the same weight, they are worth UCAS points, should you be going to university they will count towards getting in (grade 6 onwards). Aside from the UCAS points they are in my opinion a great qualification to have even if you don't work in the music industry, they prove that you have patience, persistence and commitment. The only downsides to taking grades is that they are fairly expensive at higher levels and can take a lot of time to prepare for.
Which Exam Board is best?
If you're more of a serious player, would like to take your playing or knowledge to the next level and feel like you would benefit from taking guitar grades then you need to decide which board to go with. There are a number different exam boards you can take for grades in electric & acoustic guitar, including classical style, steel string, and plectrum for electric guitar. The most widely recognised of these are:
Trinity College London
RGT - Registry of Guitar Tutors
RSL - Rockschool
Although they all carry the same weight, each board has its differences and specialities, so rather than 'best', you should consider which exam board is 'right' for you. This also goes for difficulty, each board will have relatively the same difficulty as many of them work together to ensure this, for example- if you mainly specialist in rock music for the electric guitar, then RSL's Rockschool will probably be easier than the RGT exam.
The Trinity exam has more of an emphasis on improvisation and uses mainly classical pieces, you have the option to take the classical or plectrum exam. They tend to offer more of a freedom of choice allowing you to play to your strengths. I've found that RGT tends to focus more on chord & scale knowledge, and improvisation, offering exams in electric, acoustic and classical guitar across a wide range of styles including classical, rock, acoustic and jazz. I've found this is an excellent option for budding jazz guitarists looking to gain in depth knowledge of guitar theory.
ABRSM has a fairly even distribution of playing pieces (3), scales & arpeggios, sight reading and aural tests, also with mainly classical pieces. Most people would consider this the most prestigious and 'hardest' exam board but as stated it all depends on what type of guitar player you are. Rockschool is a little different to the rest as there is a 'performance' option which allows you to play 5 pieces and nothing else instead of 3 pieces, and a range of other assessment similar to that of ABRSM. The pieces you can play (should you chose to play the pieces from the book) are more contemporary including blues, pop, rock, funk, and more fun modern styles. It's very performance driven and is great for musicians in bands, or looking to play live.
So there you have it, pinpoint your goals and interests and if you feel that you will benefit from taking grades then decide which one to go with by looking at the syllabus of each board, including the exam structure, the pieces, and the additional knowledge you will be learning along the way. These is also the question of deciding which grade to start at, generally speaking grades 1-3 are fairly easy beginner level, 4-6 are more intermediate, aimed at guitarists of 6 months+ depending on your rate of learning, and grade 6-8 are where it gets more advanced including a fairly deep knowledge of the guitar, music theory and a high playing ability. You can start on any grade but consider that if you try to jump too many grades and fail, then that money is gone, whereas closing a grade a little lower than your ability will give you a better chance of getting an albeit lower grade certificate.
Thank-you for reading.