Here are my Mar's thoughts so far.
1. Ult is extremely OP. Locking a group of enemy heroes in place would be strong by itself, then you add the damage as they go to the wall edges and their health just seems to melt. Combo'ing up with other heroes is going to get pretty silly. Monkey King? Enigma? Jakiro? Shadow Fiend? Witch Doctor? Riki? Sand King? Magnus? Dark Seer? CM? Leshrac? Veno? Earthshaker? Medusa? Kunka? BKB lets you walk out of it, so that and heroes like Lifestealer, Juggernaut, and Pango are going to be the counter picks.
2. Bulwark. His frontal damage reduction is great against harass offlaners and supports. You can have your back turned when their missile attack starts then turn into it right as it is hitting you. It works on towers. It will probably be a shut-down counter to Legion and Ursa.
3. His mana and mana regen are weak. You are probably going to want to pick an item that fills the need. Seems like you are going to have to go Battle Fury or Echo Saber to compensate.
4. Larger than normal attack range (250). Kind of feels like Monkey King (300), where you are melee but have a range to work with.
5. His Talents are underwhelming. At least levels 10 & 15, where you get to choose between +20 Move/ +8 Str and +8 Armor/ +35 Damage. With his frontal damage reduction, not sure why you would need the +8 armor.
6. God's Rebuke, the shield slam is great. Low mana cost, big damage, AE to help farm while damaging other heroes. It scales to the end game because it is a crit multiplier on your damage. The knockback components is not a stun and cannot interrupt TP's, etc.
7. Spear of Mars. Big damage, AE, lockdown, and stun if you pin them against tree/building/wall. It synergizes with your ult, pinning them to the wall doing upwards of 250 damage every second or so. It has the skill component that you need to land it on the target you are throwing at. The other draw back is that it can carry your target a great distance away from you and as a melee hero, you are going to be using precious seconds to run after the person you just skewered before you can even get any hits in on them.
Conclusion
If Monkey King had a Strength-based brother, it would be Mars. More durable, great at forcing the team-fight, has a spear instead of a staff. I suppose people will play him as a carry, but I can see some Offlaner potential. I think an early Perseverance in combination with Bulwark and and God's Rebuke to ensure some last hits/damage should secure a decent laning phase.