Answering purely from a personal point of view:
Got ADR and its opened up a nice gig with trunking pallets overnight on class 1 which requires it so was worth it personally. A lot of pallet companies need ADR people for one thing. Also used it for example on easy job doing medical bins for hospitals (wheelie bins).
Would get all modules except 1 and 7 as employer would likely pay for those since they need specialist trucks etc. Gives the best range for sny type of work for little extra money.
Course is intensive but if done right you’ll find you remember it even if it feels initially like doing all school exams at once. Tests are actually light relief. Get plenty of sleep.
Dont start trying to learn it before hand as you’ll just confuse yourself.
My cert took 4 weeks from memory via paper version. Filling in the forms for the exam board was more stressful than doing the tests, genuinely. 
Finally little tip esp if you do it with Peter Smyth - instructions on writing 2017 mist be kept “close at hand”. You’ll get brownie points for knowing that.