What a great place! I moved here after staying at the Hotel Recoleta for a week. Hotel Principe was only 5 pesos more per night than the Recoleta had been at 55 pesos (less than US$20) and was infinitely better. (Update from 2004: it's now 60 pesos per night).
There are three rooms per floor (seven stories total). Rooms that end in one or two (i.e., 71, 72) have windows facing the street. Rooms that end in three are in the rear of the building and offer more quiet. Laprida isn't a horribly busy street, so noise shouldn't be that much of a problem anyway. All the rooms have Italian-style roll-down shutters that block most light. My room (73) was very large, had a double bed, two nightstands, several windows with an interesting view of other buildings across the way, a small dining area with a round table, a kitchenette with a granite counter, a small fridge and a sink. The bathroom was medium sized and had a bidet.
The front desk people were very helpful. The man who worked at night was determined to speak English with me (to practice, I imagine). They serve free breakfast from 7-10am daily in the lobby or you can have it delivered to your room for two pesos. Check-out time is a very early 10am. There is cable TV with English and Spanish channels (over 40 channels). There is a restaurant on the ground floor and an elevator. They only take cash.
The location is superb for a tourist because it's within two blocks of Avenida Santa Fe, which has a subway station, good restaurants, ATMs, internet cafés, etc.
Everything was in good condition and I warmly recommend this hotel.