There is one good reason to stay in the hotel: location, location, location. It is perfectly situated in the heart of Palmer Square and within walking distance of several shops and restaurants.
I have been in the hotel several times (all for business). My review may be a mix of several of these visits.
Reception. In all honesty, it is not consistent: I have been there that I was helped as one can expect from a decent hotel but I also met reception staff that seemed quite reluctant to help me. One time when they could not find the reservation in my name, the message I got was basically: we have no booking, please leave (in other words ofcourse), my suggestion to look under a company name was met with great reluctance. They did not find any. I suggested to check my secretaries name and that caused a deep sigh of the desk clerck who should not be in the Hotel business. Indeed it was under my secretaries name, a practice that is not uncommon with business travellers. Lobby is nice and light though (in contrast to some other area's of the hotel)

Rooms
There are different size rooms and having stayed there with collegues I was surprised to find that some had a sizeable room and others not more than a somewhat extended broom closet, all for the same price, Make sure you know what you get. The rooms itself are ok, albeit a bit dated, but one complaint I have is the lack of ligthing in the rooms, It is just at a very low level.

Downstairs the room has some tables for drinks and food and it has a bar + an annex for breakfast.
I found the downstairs with the exception of the breakfast area quite dark and a bit grimy. Not that I mind, it gives a certain atmosphere. According to local legend, Brooke Shields (who went to nearby Princeton University) has carved her name in one of the tables (and is roumoured to subsequently have lost her virginity in one of the hotelrooms).
Food in that sitting area is quite good, not exceptional, but good.

Breakfast area
Nice view to the outside, good breakfast but one time upon paying when leaving, I got into an argument with the cashier as she insisted I'd pay for the other people that had been at my table and that had left. I did not know these people and even if I had known them, I am not responsible for their bill.
Actually it got quite ugly eventhough I just told her it was not my responsibility. When I wanted to leave for a moment I thought she would get physical.
Other times the staff has been nothing but friendliness there, although..... I know rooms need to be cleaned daily but is it necessary to just leave vacumers spread out all over the corridor even when you see guests with suitcases approaching? A sharp contrast with e.g. the imperial hotel in Tokyo where upon spotting a guest housekeeping switches off the machines, pulls them to the side, stand in line and bow)

Area
As said, a nice area especially when the cherry trees are in Bloom. There are some 'quaint' shops around Palmer square and Princeton being a student town there are also a hoist of restaurants and some bars around (Try the 'Arms and Barrister') that is close by.

If you come by car, there are parking garages in closeby Hulfish street and Chambers street. The one in Hulfish has a deal with the hotel.

Eating
Though the food in the Hotel is OK, you are better of going to e.g. The Alchemist and Barrister in a street between Palmer square and (I think) Witherspoon. A bit more upscale is La Mezzaluna (very good) in Witherspoon or try Japanese (Ichibana) in a street behind the hotel.

Summary
Definitely a place to consider when u are in Princeton, especially if you are on foot. It is nice to have a business dinner and afterwards just to walk to the hotel rather than have to go in a car.
In my experience, the friendliness of staff is OK, but they obviously have had some rotten apples there that I hope were just temporary staff.
All in all, if you are on foot: agreat place to stay, if you have a car available, there are other options (mainly around Route 1)

My family and I stayed at The Nassau Inn over the Christmas holiday. What a wonderful experience! First of all the hotel is surrounded outside by lovely shoppes and restaurants and a beautiful Christmas Tree filled with colorful lights. We so enjoyed this because it filled us with the Christmas spirit. Inside we found a warm and cozy atmosphere; fire places, red leather chairs, beautiful decorations, and a staff that all made our stay comfortable. I can't speak for all the rooms but our suite (a family of 5) had wonderful fluffy beds and plenty of space. I particularily liked the large windows that looked down over the streets with people shopping and gathering. I would definitely recommend this hotel and I hope to visit again.

The Nassau Inn is perfectly located on Palmer Square in downtown Princeton. The oldest part of the building faces the square and adds to the charm of the setting. But much of the hotel is sadly dated and needs a serious makeover; the owners seem to feel that as pretty much the only game in downtown Princeton and with presumably good occupancy rates they can afford to under invest. But seriously: upgrade these rooms and come up with a fresh language! The restaurant is great for breakfast, and fine the rest of the time. Don't miss the great Yankee Doodle Dandy mural in the bar!

I stayed here two nights while doing business at the University. The hotel is perfectly situated in the downtown area of Princeton. You are right next to the University and there are many shops and restaurants around. I didn't have a car and that was not a problem.

My room was, from what I've read in the reviews, one of the remodeled ones on the 4th floor. It was a very good size, with a comfortable king bed. There was one chair with ottoman (which I didn't use because I wasn't in the room very much), flat-screen TV, and a bathroom in good condition. I would have liked more shelf space in the bathroom. All fixtures worked properly. The heater worked well, although it was a bit noisy.

Service was fine. Check-in and check-out were efficient, and the front desk kindly arranged a taxi for me when I needed one.

The fitness room (which they bill as a "cardio room" has a treadmill, 2 ellipticals, and a stationary bike. Plus a mat or two and exercise ball. The bike was not working well - seemed like it needed lubrication. The treadmill was in good shape and did the job.

There is complementary coffee available in the lobby all day. The common areas are beautiful, as is the bar. I had room service breakfast which was delivery speedily and was quite good.

I had dinner in the restaurant across the street, Teresa Cafe (or something like that) and it was an excellent meal. I also grabbed ice cream at The Bent Spoon, across the square. Great ice cream!