A while ago ramming was very powerful, it worked on a 1 to 1 damage ratio. If you have two ships (no shields), one is 800kT, the other 700kT, and the 700kT rams the 800kT, the 700kT was destroyed and the 800kT drops to 100kT... Now I believe it is only 40% (with the above example, the 800kT would drop to 520kT), and both ships can survive a ramming attempt (not quite sure how that works though).
Ramming is still a viable strategy at certain points, such as the colonizer example given, or if you have Organic Tech. A ship loaded with Organic Armor will usually destroy a ship of the same size class, and have enough left over to completely regenerate in a few combat turns. The ship warheads are, IMO, largely useless. The kamakazi fighters look interesting, I'll have to try that out
