I disagree with arceus597 and Nekizalb.
Personally, I played Sage´s Training throughout Cities and consistently finished in the top 10 - usually ninth, barely missing on top8 cut...
Then, I decided to remove them in favour of other draw for States. Result? 1-5 and a rating drop of 68 points.
You should
never think of discarding Magikarps with it unless you are in ultimate trouble.
However, once all Magikarps are in the discard it suddenly becomes your favourite Supporter.
It gets rid of your useless cards and gives you the unsearchable ones straight into your hand. Granted, there is the chance of gettting a lot of good cards in the top 5, but with Junk Arm and VS Seeker you can even discard a Trainer/Supporter only to immediately get it back.
Another good thing about Sage´s Training is that it´s usable under trainer lock to search out for e.g. special energies if ever needed.
No matter what deck you´re facing, it´s still a fantastic addition to any Gyarados deck - I recommend at least 2, but 1 can be run as a tech.
Just for notes: in the European Challenger Cup, 5th place was achieved with a build running 2 Sage´s Training.
I have experienced Sage´s Training working very well, and I prefer those and some other draw instead of Poké Drawer+, a very popular but actually surprisingly inconsistent card to run.
The only thing Poké Drawer+ did to me was bring me a victory in a fun tournament, only to fail me completely at States. Sage´s Training helps a bunch even with only that one copy in your hand.