It's a great lens, no doubt.
But I wouldn't say it's specifically one of the best portrait lenses. A good portrait photographer can do a good portrait with virtually any lens short of super long telephotos (only because it becomes impractical). It comes down to what kind of portraiture and the setting/environment, to make it one of the best for one person. For example, if you're in a studio, does the MKII really do anything the F4L IS isn't already doing, with less weight and cost? If out in the environment and low light, sure the MKII will pull ahead. I don't think it's really an ideal portrait lens. I think it's good for someone that needs that focal length and F2.8 with good IS, and is ok with hand holding it for longer sessions or only does short sessions, or someone who is always on a tripod.
Anyhow, just devil's advocate. Superb lens. I shoot a 200 F2.8 prime a lot for environment portrait, but I do it because it's way cheaper than the MKII, lighter weight by a long shot, and does virtually the same thing (I'm not splitting hairs on measurement charts, the prime is sharp and fast). I absolutely think focal ranges are very useful. But I guess I subscribe to 2+ cameras, one with 35mm prime, one with 85mm prime, or 200mm prime for my portrait work. Just personal preference.
If someone gave me a 70-200 MKII, I'd enjoy it. But I'd ultimately probably sell it and go with an 85L II and a 35 ART perhaps.