I took AQA AS this year. Horrible. Absolutely hated every second of it! I used to love history and so thought i would love the A level. But then again I think it might have been because I studied USA 1865 - 1920 and the English Civil war and found them extremely boring. Also, I felt that I was doing 2 a levels rather than 1. I am not gonna lie, I hated everything about it! And don't think the reason why I am saying is is because I'm "lazy" or because I got a "bad grade". I did get an A at As. However, I guess it was just not for me. However, looking at what some of my mates studied (Russia and Tudors) I feel that I would have loved learning about it! So I guess maybe I wouldn't have minded so much had my units been different. Like you, I was told of how "much hard work" I would have ahead of me, but quite frankly I wasn't expecting it to be so much harder than at GCSE. As I said, perhaps I felt this way because I was forcing myself to revise for something I absolutely hated, so it felt much more "content heavy"...
I don't know really.. What I would suggest is that you look at the spec and see exactly what you would be learning about. If you do this and if you like what you see, then go for it! You have the right attitude anyway.
In my opinion it is great to have other peoples feedback but a levels should be about studying subjects you find interesting. I wouldn't take a subject simply because everyone tells me that they enjoyed it but I wouldn't be put off in case everyone I spoke to said it was horrible either. Thats why I advise you to do your research. If you do it and like it - go for it! If you don't then don't choose it.
I feel like you can't really predict exactly how you will feel about a subject - I thought I'd hate biology and is my favourite at the moment! So try. And good luck in whatever you decide to do!