Traveling around Thailand: To bus, or not to bus?

That is the question.

Short answer: It depends on your budget and on how far you're going.

a typical minibus

Pros of busing and minibusing

1. Price. For about $8, you can get a bus from Bangkok down to the Koh Chang tropical paradise island maybe 5-6 hours away, no reservation necessary. (The ferry across is another couple dollars.) Minibuses generally cost about the same, maybe a tiny bit more.
2. Convenience. Unless you're staying way out of the city, there's going to be a bus or minibus station nearer than the airports. There's a huge minibus hub at Victory Monument (relocating to Ekkamai soon) with basically large minivans that go all over the country, but there are bus stations and minibuses around the city, too.
3. Stress-free. Getting to the airport early, going through security, blah blah blah...instead of worrying about missing your flight, just show up at the bus station and get the next available ticket. If it's not a holiday or weekend, you're probably fine, provided it won't kill you to wait an hour or two.
4. Time...sometimes. Buses tend to run all day until late (you can find out specifics online or at the station). The trains are generally less frequent and slower. 
5. Air conditioning! Almost everything is air conditioned. Buses can vary, though, so you can ask at the station.

a typical long-distance bus

Cons of busing and minibusing

1. Slow for long distances. Sure, you can take that 12-hour overnight bus for like $10. You can also fly for $20-50, depending on the day.
2. There isn't always a bathroom. They make bathroom and snack stops, though. BYOTP (Bring Your Own Toilet Paper).
3. Safety. There have been a number of bus crashes, although when I've taken the long-distance bus and minibus in the daytime, the driver wasn't noticeably reckless. 
4. Traffic!!! Bangkok traffic is horrible. Do not get stuck in it. When my partner and I took the bus back in the evening, we had to get off in the suburbs and take the BTS train the rest of the way home.

Personally, I'm a big fan of the buses and minibuses. You can get pretty much anywhere for very little money, and it's convenient and relaxed. I haven't taken the scenic slow trains yet, and I will most certainly report back when I do.