I have done the trip many a time. The trip tends to be a younger crowd though there are people from all ages on the bus. Most of the passengers transfer off the train in SBA so you should get a good feel of how crowded the bus will be from the train crowd. The type of the bus varies but most are your run of the mill buses with reclining seats and little foot rests. The new Volvo buses typically aren't used on the Northbound overnight trip. They generally run on the Southbound trip overnight. Those tend to have larger seats but less leg room.
My advice is this bus can be sold out depending on the time of the week and year (popular with students going to/from home for school breaks of weekends) so try to make it off the train and to the bus in a timely manner to have a greater choice of seats to choose from.
You will have a meal stop at the McDonald's in King City just before 2am. The McDonald's will generally let those from the bus use the lobby to order and the restroom. There is also an AMPM mini-mart open nearby as well. The Northbound and Southbound buses generally meet here so there can be a little bit of a rush but the bus won't leave until people come out. This stop is only skipped if the bus is running really late.
If you have a lot of luggage I'd suggest bringing a little bag on board with everything you need. Depending on how you feel it can get a little cold sometimes depending on the heater but a pillow or sweater to use as a pillow would be suggested.
I haven't really had any problems on the trip. You will have a few hours to connect in EMY but the station is open to sit in.
This may not be as comfortable as some of the new Greyhound buses, but it isn't a bad trip. Wifi is hit and miss but good for most basic things. Contractor for the route is Silverado Stages out of San Luis Obispo and Santa Barbara, CA.
