I used to dive right in. But after a lot of bad experiences doing that (and a few really good ones), now I always learn what I can beforehand. If there is early access or a free trial, so much the better.
While I have come to feel this is prudent, I do miss the surprise of logging in without any idea of what to expect.
EQ1, EQ2, SWG, SWTOR, GW, GW2 CoH, CoV, FFXI, WoW, CO, War,TSW and a slew of free trials and beta tests