I've used it a couple times with a companion and, the main problem is that the feature is heavily limited and bugged.
Dropping in from supercruise usually screws it up for us. Last time we dropped in an Engineers base and, I got stuck and I didn't even see my friend's ship going down, for me it was standing still. We mostly get disconnected though.
And when we are not getting disconnected, there's little to do to earn money. Bounty hunting in res zones or conflict zones, and that's pretty much it. He's a trader and I'm an explorer so, it's not something we usually do. Mission rewards should be somewhat shared with multicrew, with limits like having to be present from start to finish (I wouldn't mind a 50% for each player, you are the one who opens your ship to multicrew after all).
Options are limited as it is now. You can't deploy your friend in a scarab and remain on your ship or deploy your own scarab, a shame since that feature would be great. Gunners in supercruise are hopeless. They could be usefull scanning ships and planets in 360 degrees, having their own waypoint to guide the pilot, helping with interdictions, checking the galaxy map to buy the commodities you need... Right now they just sit arround and wait. Multicrew can grow and be a great thing but, it isn't right now.
My ship was already ok for Multicrew when the feature arrived because most of my fire power can be controlled by a gunner without rendering the pilot entirely harmless by himself, but most people ship's aren't. Turret weapons are the most expensive type, so poor commanders can't really afford multicrew in their ships; most people don't want turrets anyway.
They should allow gimballed to be shared between Pilot and gimballed (of course, with less freedom to aim than turrets), like, if the pilot is not using a firegroup with them, the Gunner could do it.
What I don't really understand is why this feature has matchmaking. It needs a particular set up to work and some coordination. Right now a gunner joins and he either has nothing to do or does not know what he's supposed to do.
Wow you just summed up all the problems with MC... very well put ! Hey Fdev, you want to have success with he players base? Work on this !