So, I'll make a long story short. I have a player who's running a Voodoo priestess, and is (naturally) a posession tradition instead of a materialization tradition. In the rules for posession, the difficulty for nabbing a prepared vessel of any kind is much less. And it's a little harder for them to posess a totally raw, unsurprisingly.

However, I've noticed that unless I'm missing something, it's not terribly difficult for a spirit to go nab some random target and ride him. Am I missing something here? Posession spirits CAN just jump in a random member of the opposition as long as they make the rolls, yeah?

Absolutely

Yup, that's right, and the spirit controls the body completely if it succeeds in the possession. As in, the troll body pulling the trigger on the heavy machine gun now turned on his erstwhile buddies.

So a strong enough free possesion spirit could in fact have taken the body of a influential person, lets say the majority holder of Ares, and be running a Megacorp? Dam, I should have made my mage a possesion type haha.

It is possible, but such a person in general would have deep pockets to pay for protection and be under constant scrutiny by corporate security -- unlike a certain religious extremist in YotC who was possessed for a long time and caused a lot of trouble.

Regarding this, how obvious possession is?

My group has played it so that possession is always clearly visible and changes possessed characters appearance.
For example voodoo priestess orders her spirit to possess herself before fight and voila, she changes to more amazon look.
People that know her can recognize her, but the spirits presence is clear.

How the looks changes is dependant of tradition and spirit type.

Yes, possession does show, but there could be steps taken to disguise the possession, both mundane and magical.

Well, I'm glad to see I'm running it right, then. My guiding theory is that posession spirits shouldn't be any less useful than materialization spirits.

And yeah, Posession is as obvious as any Materialized spirit due purely to the fact that unless a spirit has the power Metahuman Form, it should always be clearly visible as being non-human, regardless of if it's in a person/object or materialized.

Actually, IIRC, it's either automatically obvious or a perception test with a threshold of (6-force), depending on where in the book you look for the rules...

I don't know about any change in physical appearance, but its worth remembering that loas have personalities. If you summon Guede into someone, they're going to start acting like a death-obsessed jester. You summon Erzuli into a troll tank, and he/she's going to start trying to charm the pants off everyone.

This is true, but remember you are not summoning the Loas themselves, as they are the Mentor Spirits. You are summoning their servants in the form of Task, Guardian -- or whatever -- spirits.