Negatives - Your life becomes theirs; your "principal" may or may not be an utter c0ck; very hard work with some odd hours
Positives - unique insight into how "grown-up" decisions are made; your face becomes know by the "grown-ups" (only good if you don't f*ck up!); a job out of the mainstream with visits (home and abroad) that no other SO3 would get a look-in; the chance to become the confidant of a "grown-up" (if only because you'll be with him with every waking moment).
I've got mates who loved their time as a Flag Lt (RN equivalent), others who couldn't wait for the assignment to end - it really comes down to the person you are working for. A bit of a recce wouldn't go amiss either, try and find out when they're due to be relieved, as the worse thing is to go from a top boss to an utter ******** with 18 months to go......
It depends on who you're working for. If they are currently in an ADC earning post speak to his current one. If not then he may have an MA/COS who can give you his likes/dislikes etc and a feel for his working style.
However, unless there is some huge red card identified above I would go for it. It will give you a great insight into how the Army works and is usually pretty good for your career. Don't worry about mucking it up as Generals usually give their ADCs a lot of leeway.
I did a pseudo ADC job on ops, for a US general. Essentially, I organised his diary, organised security, did PA type stuff, ran his outer office and drove him about on camp. I found it to be a good job and you get exposed to the higher ups, by dint of having to be your boss's shadow.
From those I have known do the job in the UK, you may hate your boss and he you, but you still prof from being his bitch for a year and at the end having strings pulled for you (choice postings, words given to the right places, report written by someone much higher than your usual reporting chain...).
Of course, you may be a disorganised cluster f**k, but I doubt your Adjt would nominate/mention it to you if he thought that (unless he is trying to bury you).