Opera is fast. Older versions like Opera 6 are the fastest things you will see.
I just find Opera useful for when you really need that extra speed. A web forum might load twice to three times as fast in Opera 6 on a Cable connection. But something like a Bank or javascript filled page won't work in it if it has all sorts of awkward scripts. Opera 7-8 work well with javascript, probably as good as firefox. Opera 7-8 a bit slower than opera 6 in many ways, but still faster than IE and Firefox by a good shot. Firefox ate more CPU power up on my computer than IE. I tested it with a resource meter/CPU meter. It feels like it was programmed in Javascript or something. So much CPU.
Latest version Opera links:
Opera Browser
Older versions, archived:
http://www.oldversion.com/ (Downloadable Old Version's of software)