I self studied the lot using AFT.
IMO if you don't use AFT you are selling yourself short. Systems is just lots of reading and memorizing which I found interesting. Flight Planning is easy, you just need to practice the concepts so you get fast at it. I was lucky enough to be flying a turbine aircraft and already doing most of the 'flight planning syllabus' in my daily flying.
performance and loading is very simple, just remember to consult 20.7.1b for certification requirements.
Airlaw is really just IREX all over again. If you are an instrument pilot you should be able to sit the exam without to much study.
Met is on par with CPL met apart from learning sigwx charts and grid charts. They are in your jepps/aip with instructions on how to use anyway.
HF is identical to the CPL, some say easier.
Nav is half a bit of reading and half practical application. I found the start of the second half of the AFT course notes the hardest to get my head around. Working backwards from the answers in the practice exams always worked for me.
Yes I found the ATPLs interesting and many things I learnt (such as SGR's) I use everyday.