I've done a PhD. You will start all bright and eager, determined to write the best thesis in history, and excited to learn everything about the field and advance the state of knowledge.<p>You will <i>finish</i> by being relieved to get shot of the bloody thing, desperate to move on and work on something interesting for a change.<p>If you love the field enough to cope with that, and still to want to dive in and work on the same sort of thing for 3, 5, or even 8 years, then go for it. Otherwise think <i>very</i> hard.<p>Why would you do this?