how helpful it is depends heavily on a few factors:
1. what is your level of effort?
if you're only going to be taking lessons and 2 games a week, progress will be slow. if you don't really try and learn from the lesson/reviews after the fact, progress will be slow. if you're making the committment to get lessons then you should probably put a certain amount of effort in as well
2. who are you going through for a teacher?
if you're doing some random KGS 3d bargain bin teacher, i wouldn't expect good results. if you're going through an established go professional, i'd expect better results. also make sure you get along with this teacher. just because they're a teacher doesn't mean they're the right teacher for you.
more importantly
there's a lot of go knowledge to be learned
it will take awhile before it ever shows up in your game
dont expect to take 2 months of lessons and jump up to a 5d.
if it were that easy everybody would be 5d.
for more information read this post i made:
viewtopic.php?f=10&t=11078