Leagues are great, but you really do need a 6 or 7 league (at minimum) sample size with the same deck before you can make judgments on it. For example, I played the same person 3x in the same 5 match league...what did i learn from that (going 1-4 overall); That my deck was not good against ritual storm. Does that mean I should scrap the deck? Absolutely not. For me, the best way to use MTGO for testing and personal play improvement is to make MTGO friends, preferably with similar goals in mind, and grind out matches on a mutual schedule. You play 10 matches with your deck of choice vs them on a gauntlet, then you switch. You can take notes, exchange feedback and (hopefully) mutually improve.
For me, leagues are a great way to kill time after the wife and kid pass out on a weeknight. If you catch me in an MTGO league (p3temangus) i'm likely playing hearthstone on my other screen :)