We're on the second session of this campaign, where there are now two groups exploring the same world. Somewhat in parallel with my home group . My home group is an elusive group that are never quite around. So really there are two player groups of adventurers and essentially an NPC group of adventurers (my home group). This creates yet another layer of verisimilitude. The world is very much alive and doesn't owe you an explanation. Things happen, get on it.
I'm thinking I would like a few more people in the pool, because my biggest concern is one group falters and we end up running a conventional campaign at a set time, which is fine but ends up with the same motivational pitfalls that West Marches attempts to overcome. The first session adventurers gave themselves a name, and I'll need to talk to them about not doing that. The large group are on the same mission. They are required to share information, including a magical map, and are not intended to be competitive or even separate. There should be flow between groups, with no individualized "groups" to speak of, although scheduling might result in this, as most players seem to want a set night. Perhaps when they're motivated to go on a specific adventure, they'll be willing to change their personal schedules to come on an off night.
So what am I running? The Colville style would be to have a large sandbox with some preset towns and adventures out there, often of the store bought variety across every edition. I will do that one day. Instead, I've got a hex crawl where most adventures are short and either home made or modified from one shot adventures. I have a lot of experience with hex crawls and understand their pitfalls and limitations, and so far it's going well. My concern with long adventures is groups go down rabbit holes and now they're by default a member of a separate group, as they're out of commission for weeks of real time. My hex crawls tend to be intricate webs of interconnected groups, all of whom think they're the hero of the story, yet none are very heroic. There's a lot of politics interspersed with monsters and treasure. This grayness means picking sides is not so easy, and defeating one enemy is to by default choose to side with another.
Another down side I see with West Marches is the campaign isn't tailored to the characters as I would do (and am doing) with a home campaign. There are eleven players with eleven backgrounds and I honestly can't put a lot of that information into what's essentially a pick up campaign. What I need to do, and I haven't expressed enough I think, is attempt to get them to align their characters with the world, rather than the world somehow serving their back stories. If your back story doesn't match the world, perhaps it's not a good back story? Or as Colville would recommend, keep character back stories light and be on the look out for a concept or issue to glom onto as your motivation. It's much easier to decide you're a revolutionary with the colonists than insisting the DM allow you to find your missing sister, kidnapped by hobgoblins.
Wish me luck! There is plenty of time for this to go off the rails.