In short, there are a ton of flaws with Greyhound but it's great when it takes you directly to where you need to go.
ALWAYS buy online to avoid the ticket lineup, although this year the service fee has doubled to $3. Their system is so old and really needs an upgrade, I'm not sure why the US site is in such a spiffy state while the Canadian site is still versions behind. Also, you cannot buy tickets online using Safari because location auto-fill doesn't work.
The busses typically do not depart on time because they are often driving in from other locations. No real time location services so you will never know how long you have to wait or how late you might be getting to your destination. (I am also bugged out that Greyhound transit still isn't on Google Maps yet but that's a different story). I've taken the K-W to/from Toronto many times - it's always been a very frequent line and it's definitely improved over the years.
My tip is to get to the station 30 min early because if you don't get on that first bus, you probably have to wait 10 or so more minutes for the 2nd one, plus you'll still have to go through all the in between stops. Before, they would be lenient with letting you on with a same day ticket with a different time, but apparently they are finally being more strict with it.
I have done a roundtrip to NY in the past and I really don't recommend it. It's the cheap way but don't even think about being able to do anything the morning you arrive. It's impossible to get sleep on there and for some reason they often use the older busses for these trips. This bus tends to be really popular, too, so it's best to start waiting in line 45min early if you want to get seats close together within your travel group.
The bus drivers themselves tend to be great and informative. Some are so kind they drop off passengers closer to their homes so they don't have to bus or taxi home from the main station.