Good info on lathes if you read the whole thread:
From the info in there, it sounds as if diamond bits are not really all that. Copper is soft and easy to cut, and HSS or carbide actually cut just as well as diamond, just diamond stays sharp longer...but can chip and destroy an expensive bit.
From the sounds of this, and talking to some of the machinists we use at work, they both seem to agree that someone with little to no machining experience is much better off with HSS or carbide bits which won't be totally ruined if you set them up incorrectly. HSS and carbide tips are very robust and more forgiving to an inexperienced machinist. Also, when you can get them for $5 or so, you'd have to be cutting a LOT of comms before you'd be getting a better return on investment with a diamond tip.