I stayed at the Don Carlos for two weeks from 4th to 18th July 2009 with my husband and son and a friend's family. We had two apartments facing each other on the 5th floor (i think, only one floor above us) and with sea view.
The hotel does have some flaws, a very slow lift, some fixtures malfunctioning (in my firend's apartment you could hear the radio playing next door through their own radio - not nice at night!), cooker quite unsuitable for a long stay (two small electric rings)? However, these were only minor problems, as far as we were concerned.
People were very friendly and always tried to solve problems (one day my cooker stopped working and they offered to cook our food in their kitchen while ours was being fixed).
The apartment had one living room with kitchenette, one bedroom and one bathroom with bath tub. The tub itself is quite awkard to get into (specially if your are giving a bath to a three year old) and dark.
The kitchen is well equipped (apart from the small cooker) with uitensils, toaster and microwave.
The beds are rather hard but they changed mine when I asked and after that it was ok.
We often had lunch either at the cafeteria (where you can smoke so only if there was nobody smoking) or the restaurant and the food is very good.
The hotel is perfectly located near the casino, the nicest area in Santander, though a long walk or a bus ride away from the center of town.
On the whole I would recommend this hotel, in particular if you have kids or are staying for a longer period.

We (couple with a child of 2 years old) spent 5 nights in an appartment at Don Carlos and were pleasently surprised! The hotel is in a nice district of Santander, El Sardinero and is located near the beach. The hotel has a garage, were we could park our rental car. The appartment consists of a big livingroom with kitchinette, a bathroom and a bedroom and is well equiped (from tv to microwave oven). We had a nice view from our room of the little park at los pinares and could even see the sea from our balcony. The rooms were very clean, the beds were made every day and the bathroom linen was changed every day as well. The beaches at Santander are lovely, but the city is not a real beauty. We went to Cabarcéno Park (stunning) and to Santillana del Mar for example, both only half an hour drive from Santander. A downside? Well, if you are looking for modern, sleek appartments you will have to look further, the furniture is very 80's and the bed has seen better days. And the breakfast is only a cup of coffee with toast or a croissant. But altogether... would definitely recommend this place if you were looking for a clean and spacious place to stay in Santander!

I absolutely recommend this Hotel. Very clean. Spacious rooms. Satisfactory equipped kitchenette.
Spacious, well supplied bathroom.
Nice view from the balcony.
Located 300-350 m from the beach. In the most aristocratic neighborhood of the city.
5-10 min to downtown in a bus.
A very good place for people that look for a good nd well located apartment without the need to clean it.

We turned up at the hotel in the pouring rain.
Booking in was quick and reasonably cheap at 60 euros per night.
We were on motorcycles and it was a bonus that they had a garage (which they did not charge to use!) BUT the ramp into the garage was exceptionally steep and the garge floor was tiled!!! Wet tyres and feet; almost a disasted!

The rooms were very spacious, clean and the location was quiet.

The bar was handy and the staff very pleasant; kept filling us with food with every drink!

The reatuarant was excellent with a good menu and great service.

The reception staff were helpful but could have been a more cheerful.

Handy for the beach and more importantly for the India Restuarant round the corner, Ghandi's - Excellent!!