We really enjoyed our stay at 42 Castle St. As a big BeaTles fan I found the proximity to all the BeaTles related sites to be my favorite feature. A short walk to the Cavern club, and the statue at the dock. If you wanted to do the Magical Mystery tour, it would be a short walk to the pier for that.
We had a car, and took advantage of the parking discount, and that worked great! We were able to get around to all the other sites (Penny Lane, Strawberry Fields, etc)
There are are plenty of options for food and drink within walking distance. We enjoyed every one we visited.
The apartment was clean and well appointed with everything we needed.
I would definitely stay here again.