For around $30 one way, you really can't ask for much, they generally leave and arrive on time, and I never really had an issue with them.
Some tips I would suggest..
1. Arrive at least 30 mins before departure, they sometimes oversell the bus and you would be left waiting for the next one.
2. sit in the upper deck, I find there is more leg room there.
3. Buy your own food before you go
4. Use the toilet before you board the bus and during the stop over at Kingston if possible, the toilet on the bus is pretty bad.
5. This is the most important... DO NOT EXPECT 5 STAR SERVICE, as long as have a realistic expectation of the experience you should be fine.