Everyone's description of logistics is pretty accurate. I'll add that it's very feasible to do this adventure by arranging a guide by yourself (as opposed to a guide service for ~$100/person). It's possible to just show up at Pasar Agung Temple at 1:30 am and negotiate with a guide (expect to pay $40-50 for 2-3 people) to take you up that morning. If you're like me and want to have a little more of a sure bet, contact a guide ahead of time. There is a network of 28 guides who all know each other - 21 men and 7 women. I highly recommend Wayan Widiyasa (firstname.lastname@example.org, +6285237250607), who speaks awesome english and can handle slow hikes or even people who want to blitz ascent. Try to catch him a few days ahead because he's popular. If he's booked he can hook you up with a family member - they're all farmers who serve as guides in the dry season. If you want to arrive the day before, ask your guide if they can find you a place to stay. Most of them seem to have an extra room with a mattress on the floor where you can crash for a few hours. And they'll most assuredly want to feed you and serve you tea. Amazing people with awesome hospitality.
My wife and I are in good physical shape and are avid hikers in our late 20s, and this is the most intense hike we've done in terms of steepness and ruggedness of terrain. It didn't help that we did it in Chacos - you'll survive in sub-optimal footwear, but hiking boots and hiking poles would be VERY nice. Believe the reviews that say you're basically rock climbing for the last hour or so. Also, take every bit of warm clothing you have and wear pants on the hike (NOT shorts!). It's near freezing and windy at the top.
And a last piece of advice: tip well. These guys and girls work hard.
Okay, one more piece of advice: DO IT! It's so worth the work and investment. My wife and I are big-time adventurers and this is one of the coolest things we've ever done!