No, I haven't heard of NH Salzburg but I assume they're part of the Spanish NH Hotels chain. I have stayed at one of their properties in Barcelona and it was great.
I last stayed at Krone 1512 in September and had a very comfortable stay. The service was great, especially at breakfast. The bar adjacent to the front desk can become very "smoky" in the evenings, but that wasn't a problem in my room. They have a nice back garden where you can sit if the weather is nice. The location is perfect and it's an easy walk across the bridge to the old town. There are some nice restaurants within a five minute walk from the hotel (including some on the same street as the hotel). I was able to walk to the rail station, but with luggage using a Taxi is a better idea.