"I can't beat that. I guess I scoop?"
-Astounded DS player, staring at a turn-2 board full of walkers
Let's break this down. In general, midrange = good matchup, combo/fast mana = bad matchup, aggro = about 50/50, and control = unfavorable, but sometimes you can steal a win with Defense Grid and rub their smug blue faces in it.
Affinity
This is a decent matchup for us. Affinity doesn't have much that can disrupt our main game plan, and an early Walking Ballista completely shuts them down. Post-board, swap out a few Rangers of Eos for Nature's Claim, but otherwise not much needs to change. What are they gonna do, play Stony Silence?
Burn
Burn is about a 50/50 matchup in game 1, with the main threat to our deck being Eidolon, but it gets easier post-board. Hangarback is great against Bolts and Goblin Guide, and every spell they send at a Metallic Mimic is one less they send at our face. Take out the Thoughtseizes, then board in Nature's Claim for lifegain and Dromoka's Command for dealing with Eidolon (it's an enchantment creature) and preventing damage.
Death's Shadow
Most Death's Shadow variants are really good matchups for us. It's difficult for them to go low on life when there is a constant threat of big Walking Ballistas, and the combination of Hangarback Walker and modular helps us resist most of their removal. Take out some Winding Constrictors and board in a few Path to Exile to deal with delve creatures.
Death and Taxes
Tax decks don't tend to bother us very much. Our deck can function on just 2 lands, and shooting down a turn-2 Thalia with Walking Ballista is so satisfying it almost makes me feel dirty. Take out a few Winding Constrictors and board in Dromoka's Command to deal with Stony Silence and Rest in Peace. Maelstrom Pulse can also help with Lingering Souls tokens and Aether Vials, but then again, so does a Walking Ballista.
Dredge
I feel like Dredge should be a really bad matchup, but I've never had many problems with it. A lot of their creatures die to Ballista, and they don't run a ton of removal to slow us down. Even if the game goes long and they gum up the board by recurring creatures, we can still go over them with Thopter tokens and Ballista to the face. Take out Thoughtseize and board in Grafdigger's Cage and Surgical Extraction.
Elves
Elves is an interesting matchup. They have no way to disrupt our main plan, but if we don't draw Fatal Push or Ballista to deal with their early mana dorks then they overwhelm us very quickly. There's not much to change after game 1, just mulligan for removal.
G/B/x Midrange
Jund, Abzan, G/W Value Town, etc. These are all good matchups for us. We're afraid of Wrath effects, but the targeted removal in most midrange decks is inefficient against a lot of our stuff, as most of our creatures give plenty of value even if they get Fatally Pushed. Jund is probably the toughest matchup, but it's still favorable for us. Board in Maelstrom Pulse to deal with planeswalkers and keep an eye on how close you are to winning with a Walking Ballista topdeck.
Hollow One
Ugh, we can't really win this one. Burning Inquiry screws up our sequencing and we don't actually have a maindeck answer to Hollow One. Board in Path, Surgical Extraction, and Nature's Claim but don't get your hopes up. The only upside to this matchup? I imagine that the feeling of dread I experience when someone drops 2 free Hollow Ones on turn 1 or 2 is a similar dread to what other people feel when they see free Hangarback Walkers and Walking Ballistas.
Infect
Are people playing Infect right now? Anyway this is a tough one. Game 1 is rough unless you start with a Mimic and Ballistas, and post-board you need to mulligan until you find removal and then pray. Always board out Winding Constrictor, since it adds additional poison counters whenever you get hit by an infect creature.
Ponza
Similar to the D&T matchup, we don't suffer that much from a Blood Moon or land destruction. As long as we can get to 2 lands we can usually resolve enough threats to win. Watch out for Anger of the Gods since it shuts down Hangarback and modular. Board in Path to deal with Inferno Titan and the occasional Platinum Emperion + Madcap Experiment combo.
Tron
Tron is Tron. It's linear, it's uninteractive, it's fast, it usually beats us. We do have a few tricks though - if you can get down Mimic with some walkers on turn 2 then it's possible to race them, and if Arcbound Ravager is on the board then we can use it to manipulate counters and save our Hangarback Walkers from Karn and Ulamog. Oblivion Stone and Wurmcoil Engine are the real issues. Take out Fatal Push and Ranger of Eos, board in Ghost Quarter, Nature's Claim, and - if you're feeling lucky - Surgical Extraction.
Titanshift
This is probably our worst matchup. Just play fast and pray they don't have Scapeshift. Board out Fatal Push, then add Ghost Quarter to deal with Valakut, the Molten Pinnacle and Path to Exile to remove Primeval Titan.
Storm
Storm is one of our worst matchups. The only hope we have is to go for the turn-4 kill while playing around Remand and hope we have Fatal Push for when Baral comes down. Take out the Rangers of Eos and bring in Surgical Extraction and Grafdigger's Cage for games 2 and 3.
U/W/(R) Control
Blue is just tough. We can't do much except play around countermagic, and Cryptic Command hits us extra hard if they bounce a creature with a lot of counters on it. We're also vulnerable to Ceremonious Rejection and Path. Board in Defense Grid and hope for the best.
5C Humans
Humans is one of our most interesting matchups. Their creatures get pretty big if we don't draw removal, but we can block all day with Hangarback Walker, and they don't have much removal to screw up our synergies. Our main concern is Mantis Rider and let's face it, that card is bonkers. If you can make it to turn 4, odds are they haven't removed your Metallic Mimic, so you can play Ranger of Eos and then cast 2 Hangarbacks to immediately stabilize. This matchup actually gets easier post-board: bring in Maelstrom Pulse and Path to Exile to help deal with their evasive threats, and Nature's Claim if you're worried about Aether Vial.