Already been stated here but I'll add a little...Jailbreaking will not fix 2.1 issues (crashing, lagging, etc) but does add a lot to the phone in general. Themes are great, apps are abundant and its great fun to "mod" your phone to your liking. I do have to disagree that Jailbreaking will make your firmware work worse. I think it depends on what you add through Jailbreaking rather than Jailbreaking itself.
With said, as Confuzed indicated, use Quickpwn to Jailbreak.