now that we have the rules and have seen previews for every faction I believe we can finally start talking stratergy

I find mobility to be one of the more interesting special rules so far but im not sure how best to use it. At first I thought of deploying a mobile unit on an empty planet (or 1 where the enemy has less command icons to the mobile unit) next to where I think there will be a battle then moving it to the battle to fight. This would basically give me a discount on the unit.

my second thought was to combo it with promotion, avoid combat and win 0-2 cards and resources per turn. The problem with this however is that it takes 3 turns to pay for itself (vile raider is a card costing 4 resources and promotion is card) so unless you are also denying your opponent resources they would otherwise get I don't think it is worth it.

what do you guys think is the best use for mobility

I think mobility gives something incredibly important: flexibility.

Want to make sure you win the command struggle at a planet while still being able to support your Warlord in combat on an adjacent planet? Want to win the struggle with minimal risk to the unit? Want to bluff which planet you're going to commit the Warlord to, and keep your opponent guessing?

Mobility. It's gonna be strong I think.

I think there are two big uses for mobility, one direct and the other indirect:

1) Direct: As you say, help win a command struggle at one planet, then help win a battle at another.

2) Indirect: Bluffing. You don't, after all, HAVE to move the mobile unit. You can leave it where it is. This puts all kinds of pressure on your opponent when deciding where to send his own warlord (and thus units in HQ) at the beginning of the Command struggle. Does he send it where the mobile unit IS? Where it MIGHT go? Don't underestimate the power of a "moving part" on influencing the warlord commitment part of the game (a huge psychological component of the game, I think).

so unless you are also denying your opponent resources they would otherwise get I don't think it is worth it.

This is why I think your original plan of sending it to an unoccupied planet for an easy command win is not the first strategy I'd employ. I'd much prefer to send the mobile unit to a planet so as to have more command icons there than my opponent. I'm probably not getting much more than I would have at the empty planet, but I am making sure my opponent gets less in the Command phase. Any time I get resources/cards and my opponent doesn't is a "good time" in my book.

The fighting may take place in the Combat phase, but I think the real competition is going to be in the Command phase.

The Tau Crisis Battleguard look like the best unit for these purposes, 3 Command and mobile. One of my favorite tabletop units so I look forward to fielding the card.

Mobile will be huge, it's like you can be two units in one! And yes, the Crisis suits for Tau seem like the epic example of Mobile! Go Tau!

Mobile is potentially powerful but I think it's a bit expensive at the moment. I expect it to get better with more expansions.

It is definitely a tricky ability, and one that is not as straightforward as some of the others. I think the purpose is, like stated, to win command struggles and stay relatively safe from being singled out, allowing you to build up a board presence without having to lose units in the process. This can either be used to bulk up on adjacent planets that are having a combat, or to retreat from warlords that are trying to divide and conquer the generally weaker units that have mobility.

I feel that mobile units are best when deployed to a planet that your opponent is staking some claim to...in the Crisis Guard situation i would depoly them to a planet that already has an enemy presence of 1 or 2... that way you effectivly nullify their unit, and if they send their warlord to that planet, you can mobile away rather than die.