It does seem to be a personal thing; whether you sleep hot, cold, move about much etc etc. Your choice of mats makes a big difference too, the most recent air filled (no foam inside) tend to be cooler.
Using myself as an example, I'm pretty average build and although I apparently make a good hot water bottle, I do tend to feel the cold at times. I sleep starkers, even on top of a mountain, so do tend to need a warmer sleeping bag than some.
I have three sleeping bags; one for very hot weather, one that works well almost all of the time and one for when I decide to do something daft in the winter. Over time, I've found what temperatures I'm happy with in each.
Generally speaking, I find my own opinion is more or less in line with the manufacturer; I avoid using each below (roughly) the comfort rating. The exception being the winter bag which only had a limit rating of -25 (the comfort and extreme both say n/a) - I used it a few days ago in -3 or thereabouts and would now say, if I want to use it below that, I'll have to stay dressed. Even then, I don't think I'd want to go lower than minus 10.
Personally, I'd say 5deg comfort won't be enough in early spring, especially up north. And it seems a little heavy for what it is. But, you may find you're happy with it. I'd use a campsite the first time you use it and take a duvet or blanket to put over yourself if you find it's not enough