Valkyrie is basically a "harder" version of the Premium server.
For example, rates are lower, some items in NPCs are sold at much higher prices than in the Premium server, etc.
I say it's playable if you have people to actually play with, it makes the experience more enjoyable.
As for jobs, it really depends on what you like. Do you enjoy fighting face to face? Running away and casting? Supporting? Tanking? Etc.
For raids, there's a thing called WoE (War of Emperium) where guilds fight for a castle, it's really fun if your guild is actually decent.
Bosses are hard to come by since everyone kills them, but they usually require a party to take down.