So, before I even received my first Japanese course book from JET, I was told it was rubbish. But, after 3 books of the beginner's course, I've realised they are quite good.
Many said they were not very helpful, and there are a few things the beginners course could improve on, but overall it teaches great Japanese. Some of the bad points about the beginners course is that it uses romanji instead of hiragana and katakana. It would really help if it dropped you into the language fully. I mean, it could of even thrown in some of the beginning Kanji to help out. Saying that, they do send you an additional (separate) hiragana/katakana/kanji book. It would have been great if they used it in the lessons though.
But that aside, I still think it is overall worth the effort to do. The vocab and grammar it teaches is great. The exercises are great.
(the friggin voice on the CD drives me crazy!!!! Honestly it is recorded torture...small annoyance, had to bold it though. You'll understand.) I'm only 3 books in and it is teaching some really helpful grammar. Although, I've noticed, it is very hard to remember the vocab! I've done other courses (Japanesepod101) and it just sticks, but here it just runs and hides. So I recommend downloading
Anki (flash card program) and turning all that vocab into flash card goodness. Anki really is a great flash card tool, free too.
As I said, I also study using Japanesepod101. This is where most of my Japanese knowledge has come from. If your serious about Japanese, I recommend you also study it else where aside from the JET standard issue course. You really should be tackling kanji, hiragana, and katakana head on. It will make life easier in the long run.
Any way, end of that. Do the course! It does have some good fruits in it. Make sure you get the appropriate level for yourself!