Like most things, you've got to blend techniques. I've worked a lot from life and a lot from photos. Digital photography has allowed me to paint and draw a lot of subjects that I wouldn't have access to any other way. Most of my pet portraits have had to be from photos, but I always try and meet the dog to get a sense of their coloring and spirit. With both, there are advantages and disadvantages. Photos are good for setting the pose, but the coloring seen in a photo is completely wrong, especially is the subject is white. Photos bleach out a lot white and also a lot of flesh. A computer screen only has access to a few pigments, but artists can choose from hundreds of pigments.
I'll never know what works completely, so I can't offer surefire solution to. For me, I know that photos can work well if they're clear, but a blurry photo won't do you any good. There's also the problem that photos don't show a continuous time frame as if you're painting a living and breathing subject. The best colors and feeling come from seeing nature directly.
Photos are always one step removed from life so they'll never be as powerful. It's like reading a translation of a great work of literature. I read a lot of Murakami, who writes in Japanese and most of his books are translated into english; I always feel like I'm missing something from the original Japanese. Some words and concepts don't translate over through languages. The same thing with listening to live music or a recording. Live music may not be a great representation of the sound, but the fact that it's live trumps a 'flawless' studio recording. I'm don't know a lot about recording music, however, I feel that studying one art form reveals the secrets of all of them.
The same principle holds true for painting from photos. Any trained eye can tell when a work is lacking the feeling of working from life. I feel like a lot of artists gravitate to photos because it seems easy to paint from a photo; this is because the image is already 2D so you don't have to think about turning 3D into 2D. You could take the easy route with painting and not work from life, but your work won't ever amount to anything.
Paint, sketch and draw from life until you learn how to use photos in terms of color, edge quality and texture.