In FS9 I used to fly on-line all the time, either on the Emma Field server (thanks Francois) or on occasion when that was down, on Netwings. Always with FSHost. The EFC server always used to have a few friendly people kicking around as did the Netwings server, although I preferred the EFC server as it was password protected and I wasn't asked "hey look at me landing on this roof in my 747" all the time...well except when members were drunk!
Then FSX divided the community and the servers grew quiet. Wheras before you could just fire up FSHost and Teamspeak and join a like minded bunch (of nutters usually), any MP event had to be planned otherwise you'd be on your own. So after a short while you stop dropping by the servers.
FSX - Gamespy. I took part in my first MP session in quite awhile on Sunday afternoon with Ian Pearson. We loaded up Gamespy and ran the L-39 in shared cockpit mode. I certainly had a blast. However, I had the sim set up to start Multiplayer in the Bushflying room where it's much quieter, but when I swapped to the Freeflight room afterwards to see what was going on, I was almost immediately kicked out. It took me 15 attempts to get logged back onto MP long enough to switch to the Bushflying room where it seems more stable.
Now in three weeks I won't remember tooling around having fun with a mate, I'll remember the bruise on my head which occurred while trying to log onto Gamespy.
So what about FSHost? It was excellent with FS9 and most likely still is, but it never worked as well with FSX and the lag stutters made it unusable in my eyes.
So to summarise, I love flying in MP, but I've found the environment in which I have to do it severely lacking. Sad really as I've never really enjoyed flying in the sim very much since I stopped flying FS9 in multiplayer.