Pai is a predominantly tourism-oriented town, offering a relaxed atmosphere with a broad traveller & backpacker scene. In early 2006 a sudden boom in guest-house and bar construction has resulted in a great deal of spare capacity - capacity that is partially taken up by an increase in Thai people visiting after Pai was featured in a romantic Thai film.
Route 1095 which connects Pai with Mae Hong Son (50km as the crow flies, but approx. 110km by road) and Chiang Mai (135km) is a very scenic route through the mountains which takes several hours (but worth it). It's a steep and windy drive, with lots of curves, so take a plastic bag and some motion-sickness pills if you need them.
Route 1095 isn't as bad as people make it out to be. There isn't much traffic and you can hear the cars and trucks coming. If you're a little adventurous, rent a motorcycle in Chiang Mai and make the ride up to Pai. You can stop at the waterfalls and small towns along the way, and you'll really enjoy the trip, as opposed to being motion sick in a bus for hours, and being forced to stop at the driver's friends restaurants. The scenery is absolutely gorgeous and being on a bike makes you feel like part of the mountains. The locals will think you're crazy, and the construction crews get really excited when you come through. Make sure to take some warmer clothing on your bike, as it tends to get a bit chilly in the higher portions of the ride. As a novice rider, expect the trip to take around 5 or 6 hours, including stops at sites and restaurants along the way.
Buses and minibuses go to Chiang Mai (Arcade terminal) and Mae Hong Son. Regular public buses take around 4 hours and charge about 80 baht; minibuses take around 3 hours and tickets (sold by travel agencies) cost about 160 baht. One strategy is to get to Pai using the public bus so that you can get an idea as to how winding the road is and then you can decide if you want to splash out and get the mini bus back to Chiang Mai.
The nearest international airport is at Chiang Mai.
Pai now has daily service from Chiang Mai. Tickets can be bought at Aya Services in Pai, or from the ticket office in the Chiang Mai airport. It's near the Air Asia office, on the far end. Chiang Mai to Pai - SGA departs daily from Chiang Mai at 10:55am and arrives in Pai at 11:30am. Return flights are at 11:45am arriving at Chiang Mai at 12:20pm. 
The nearest train station is at Chiang Mai.
The town itself is best explored on foot. For exploring further afield, bicycles (40-100 baht/day) and motorbikes (from 100 baht/day) can be rented from many agents along the main street.
Motorbike taxis are also readily available.
The town itself has no special sights; most people come simply for the relaxed atmosphere. Nearby attractions include hot springs and waterfalls, and a hilltop temple. There is also a wonderful canyon which provides the perfect spot for a sunset. This is a great spot to visit after seeing the WWII bridge built by Japanese-held POWs.
Rent a bicycle or motorbike and visit one of the nearby waterfalls and hill-tribe villages. Pai is also a major starting point for organized trekking tours which are offered by every guesthouse and travel agent.
Whitewater rafting trips abound and there are numerous elephant camps. Additionally there are several hot springs in the area.
Take a look at some of the hill-tribe members selling handcrafts.
Pai has an abundance of bookshops, some of which carry harder to find titles. Many are along the bus stop road, past Aya services.
For such a small town, there's an astonishing number of restaurants, most of them catering for needs/tastes of foreign travellers; just choose the one that suits you best.
There are many Western-style bars, especially along the main street that leads to the Chiang Mai bus stop. There are also many tea and coffee shops, including herbal brews.
There's an abundance of guesthouses in Pai, most of them in the budget range (a bungalow goes for around 100-500 baht depending on amenities included). Mid-range options are rare and there's no top-end hotel. But Pai is not a package-tour-place.
Heading out of town there are swarms of bungalow setups.
At the bus station there is a 2007 (!) map of Pai. Get this as it will show you the location of most of the guest houses (> 100 places). There is also a discount for motorbike rental.
Pai has several Internet cafés, most on Thanon Ratchadamnoen and Thanon Rangthiyanon. They are the only example of poor value in Pai at a uniform 1 baht/minute for generally poor connections.