We will need to do this trip in the near future. No car so I believe our options are:

fly - not interested. too much hassle for such a short flight.
Amtrak or Greyhound - possible and probably the cheapest options but maybe not if we add the taxi/public transportation cost to get to either one.
limoliner.com - a luxury bus service. The major selling point is that the company's "stations" are the the 2 hotels we will be staying at at either end. It is a nonstop ride that takes about 4 hours.

How is their on time performance? Is it really that nice and is the extra money worth it?

Well, I believe you have no choice and basically, those buses are fine with tv screens, air conditioning etc. I don't see any problem to it!

I know you're not interested, but Amtrak/Greyhound are NOT the cheapest option.

The cheapest option is to get $1 ticket on Megabus. I've taken it a couple of times now and I'm still alive. My brother, rkkwan, has taken Boltbus for this route for a bit more $ (I think it was $10?) and he's still alive.

Since MegaBus & Boltbus take the same route as Limoliner, I can tell you that it rarely is 4 hours - unless you are leaving at 7am on a weekend. During the week, it's more like 4.5 or 5 hours due to traffic into Manhattan.

The limoliner is very nice and as on time as any bus service will be.

It would also be very, very easy for you to take Amtrak though. It leaves out of Back Bay station in Boston, just about a 5 min walk from your hotel.

Both options work, although you are more limited with the limoliner schedule.

I didn't know Megabus expended eastward. Thanks yk. I will look at that option.

As long as it's no more than 5 hours we'll be fine.

With road you can never be sure. I generally do Boston by car (from the upper west side) in 4.5 hours, including a 10-minute stop. But I drive faster than the bus will.

The problem with bus versus train is that you can have bad weather, construction or accidents that can cause significant delays. Delays with the train are also possible but much rarer.

my recollection is that Amtrak and Limoliner are pretty close to the same price.

I've ridden the limoliner just once, because I happened to be leaving from the NY Hilton, so it was super convenient. It was very comfortable, and a pleasant trip. Its nice that it has wifi, and they do show movies on some overhead video screens.
But all in all, not really much different from taking Amtrak, and slightly slower.

Amtrak is currently running specials for that route, as little as $49 each way with 14-day advance purchase, which is roughly half the cost of the Limoliner.

As noted above, Amtrak leaves from the Back Bay; the available buses (Bolt, Mega, Greyhound) leave from South Station, which might be a $10 cab ride from Back Bay hotels, so not a major financial consideration especially for 2 people. All the buses will take about the same amount of time since they all encounter the same traffic. All are significantly cheaper than Limoliner. Bolt and Mega offer several comparable amenities, such as free WiFi.