reasonable (if not varying) prices
conveniently located bus terminals
free WiFi and outlets if you're lucky
flexibility: if you miss your bus, you'll have no problem getting on another one at no additional cost
you never know which buses are going to make the rest area stops, so it's impossible to plan ahead
Greyhound and Peter Pan tend to overbook, so if you're at the end of a long line, you may be out of luck
Traveling to and from NYC and Boston can be a real crapshoot with Greyhound or Peter Pan. You just never know what kind of experience you're going to get.
Prices on Greyhound and Peter Pan vary depending on when you order your tickets and how you order them. When I order my tickets online at least week in advance, I've gotten prices as low as $14.
Prices do tend to go up during holiday seasons and other peak travel times. And if you wait to purchase your ticket directly at the counter, the best price you can get is around $35.
Going from Boston to NYC, the Greyhound and Peter Pan buses depart at Gate 3 at the South Station bus terminal. Although not as posh as the AmTrak terminal at South Station, the bus terminal is a nice facility with a variety of fast food options and seating.
Going from NYC to Boston, the Greyhound and Peter Pan buses depart at Gate 84 or 85 at Port Authority Bus Terminal. I'm not a fan of Port Authority. It does not surprise me that Port Authority was ranked one of the ten ugliest buildings in America. And their long lines and unhelpful customer service people do not lend any favors to the experience.
One thing you have to keep in mind about Greyhound and Peter Pan is that they tend to overbook. Unless you are willing to pay the extra for $5 for priority seating, then you'd better arrive early or you may not get a seat.
On the flip side, however, there's one advantage of Greyhound and Peter Pan that you don't have at Bolt Bus. If you happen to miss your bus, you can always take another one the same day. (If you have a Bolt Bus ticket and miss your bus, you're up a creek). Greyhound and Peter Pan offer a lot more flexibility about when you can use your ticket.
As far the bus ride itself, so much depends on traffic. On a good day, you may arrive in just over four hours. If you end up in holiday or rush hour traffic, it could take nearly seven hours.
You also never know what kind of buses you're going to get. Some of the newer buses have free WiFi and plenty of outlets for your laptop, but some of the older ones don't. You also never know if the bus driver is going to make a food/rest area stop. Some drives do, some don't. This makes planning for food and bathroom breaks a bit difficult.
The bus drivers themselves usually do their job well. Sometimes you get a witty and friendly driver who makes the ride more pleasant. Occasionally you get a few eccentric types. Once, I had a bus pull up to his own driveway at his house just to drop off his briefcase!
As for passengers, Greyhound and Peter Pan mostly attract the college/grad student crowd, though you do get couples and families with small children.
Greyhound and Peter Pan are a much better deal than AmTrak (which can run as much as $110 for a NYC to Boston trip!). As far as experience, you never know what you're going to get, but at least you know you'll pay a reasonable price and arrive safetly...eventually.