This hotel would have been perfect for me if only there had been a CityBike station nearby. But that was my own fault for not doing my homework before booking.
Somehow, I thought Vienna would be like Paris, with 1400 bike stations all over the city and nearby suburbs. But no, Vienna has only 121 bike stations, all concentrated in the city center. The Favoriten District (aka District 10) has no bike stations at all, so the nearest ones to my hotel were at the Central Train station (over one km to the north) or at Matzleinsdorfer Platz (well over one km to the west) — not close enough to be a practical option for coming home at night after the opera.
Otherwise, though, the Hotel Caroline is well served by public transportation. The bus 14A stops directly in front of the hotel, the tram “0” (I don’t know if that is meant to be a zero or the letter O) is only a block away, and the subway U1 has a station just three short blocks away at Keplerplatz.
The Central Railway Station is the first stop on the U1 after Keplerplatz, so it’s very convenient for those of us who travel by train.

My room in the Hotel Caroline
My room was in a newly-built annex to the hotel (easily reachable by a bridge over the breakfast room). The room was quite huge, very clean and modern. It had a safe, a TV, a refrigerator, a telephone (OK, nobody uses hotel telephones any more, but it was still there) and a well-functioning Wi-Fi hookup. The large bathroom had a bathtub and shower and even a hair drier. For single occupancy I paid 67, 68 or 71 Euros per night, depending on the day of the week, including breakfast, which I thought was a very reasonable price.
(Prices as of 2016.)

Physiotherm cabin in my room
My room also had its own private Physiotherm cabin adjoining the bathroom. I was tempted to try this (never having tried or even heard of such a thing before) but from the instruction booklet it sounded like a lengthy procedure, and I never quite found the time for it, since I had a full program of things to do while I was in Vienna.

Breakfast room
The breakfast room in the basement was roomy, quiet and pleasant. Breakfast was in the form of a small buffet, which was more than adequate for my needs. The only thing missing was the morning paper, which they neglected to provide.

Breakfast room from the bridge
This is the breakfast room as seen from the bridge which connects the old and new buildings of the hotel.
Hotel Caroline is at Gudrunstraße 138 in Vienna.
My photos in this post are from 2016. I revised the text in 2021.
See more posts on Vienna, Austria.
Looks like a very good room for the money even if the location wasn’t ideal for you. I would have been curious to try out the Physiotherm cabinet, especially as I see they say it is good for bad backs!
Yes, I thought it was excellent value for money.
The physiotherm sounds like it might be like a sauna.
I once booked the closest hotel to the waterfront to go on a river cruise. But it was the hotel for the Viking cruises, and was on the other side of the canal from the cruise we were to take on AmaWaterways. We could walk to the boat, but not with luggage. We had to take a taxi. And the breakfast buffet was very expensive and impossible for me to get any food on a scooter.