Thoughts on Spin/Cycling?

Stavakoli

So I just started taking spin classes at my gym and I'm really loving them. I'm thinking of making that my daily workout for a while and I'm really hoping that will jump start my weight loss as I've been at a plateau for months now.
I dont have an HRM and am getting conflicting information about how many calories I'm burning in one class (50 minutes.) In that time I'm doing between 18-21 miles depending on the instructor. I am working hard and feel the burn, but the classes are just not as difficult (for the lack of a better word) as running/boxing which I had been doing in the past. Does that mean I'm not burning as much? Should I add strength training to my routine since I'm kind of neglecting my upper body? I know every body is different but any info/opinions on spin would be awesome!