As with chromoly, butting the tubes lightens the frame's overall weight. However, aluminum will remain stiff when butted, as opposed to chromoly steel, which becomes more compliant. Easton, Alcoa and Reynolds are the most common brand names. In recent years bike companies have started experimenting with other frame materials, some of which are quite exotic and expensive. Titanium has become more available as demand has increased.
Fabricators can then order special tube diameters to better facilitate bicycle designs.
It is still a fairly rare material, and is finicky to work with. The ride is much like a good butted chromoly frame, but with even less weight and better durability. The interesting thing about titanium is that there are only a few major manufacturers. So if you see a bicycle frame made of titanium, more than likely it has been build by one of two or three makers. This helps bring the cost down, especially since they are built in larger numbers.