I went on the Epic last year with Cruisemates. The cabin I had was one of their special solo cabins. While the cabins are not huge, they still fit the bill as far as not having to pay twice for a cabin. Everything was concise and yet I didn't feel cramped. There was enough room for all of my clothes. And the shower was big enough for me. That is a real plus because I am a big guy. Often on cruises the shower is too small and too tight. But not on the Epic. I had plenty of room to turn around in the shower. And the soap came out of a dispenser which helped. There even is storage space under the bed (queen size bed). One of the features of the cabin I liked is you can program the lights in the cabin to gradually change colors. Also, the entry way to the cabin area is locked. So only those people who had a cabin could come in. In the middle of the row of cabins is a gathering area just for those cabins. There people gather in the evening to discuss what the next day is going to bring. Sometimes people in the solo cabins made arrangements with other solo members to get together for a meal. In the morning, the gathering area has continental breakfast for those who just want coffee, juice and a roll.
I think not having to pay double for a cabin is great. So is having a cabin to youself. My opinion is go for the single cabin--the week I went I didn't hear any complaints about the cabins at all.
If you have any questions, I would be happy to answer them.