Sawtooth Loon is looked down by most of the magic community. If I pay 4
mana I get a creature and 2 cards in return. But this is a good thing in a
few ways:
1. Yet another Honorable Scout combo.
2. The cards are free, since you don't get any closer to decking yourself.
3. Replay Meddling Mage.
4. Get more counter-magic.

I've tried it out in a W/U Magpie-Go deck, and it helped a lot, like this one:

10 Plains
10 Islands
4 Coastal Tower

4 Thieving Magpie
4 Sawtooth Loon
4 Meddling Mage
4 Benalish Trapper
4 Stormscape Familiar

4 Counterspell
3 Orim's Chant
2 Healing Salve
2 Disenchant
4 Sleight of Hand

This deck focuses on early game control, with 24 lands to get ya there. The
trappers, counters, and chanters stop your opponent from doing much. The
salves are there because you can get off to a slow start sometimes. Meddling
Mage takes care of red and black (usually). Heres a sideboard:

1 Meteor Crater
3 Voice of All
3 Honorable Scout
3 Exclude
3 Story Circle
2 Essence Leak

This adds more protection against the other colors. Again, the loon gets you
the cards you need in this deck. The Loon isn't perfect for every U/W deck,
but pretty good for the sideboard.