I've been looking at those as a summer bag. Probably good inside a bivi bag even in the hills?
No, I would not use it as a sleeping bag on its own, for that it is not warm enough. I also think it won't work all that great in a bivibag (on its own). Although I use it with great succes, I would never depart home with only the Reactor.
To give you some more detail, I use a summer bag with a rating of +10°C. When I use it in conjunction with the Reactor I can sleep comfortably up to temperatures at around 3°C (in a bivibag, under a tarp).
I use a winter bag with a rating of - 15°C. I get chills when it reaches at around -8°C (in a bivibag, under a tarp). With the reactor I am able to remain very toasty at the same temperature.
I look at the reactor as a great means to upgrade older sleeping bags which you might otherwise discard. The weight/warmth ratio of the Reactor is pretty good imo.