55-250mm for a portrait lens?

I have a shoot with a model coming up this weekend, and I'm trying to figure out which lens will be best overall from my kit. I'll be using the 50mm 1.8 quite a bit, but I know I'll want to vary the look. I have the 55-250mm, and a nice older 28-70mm mkI that is really sharp (a well kept secret lens with a cult following). Ideally, I'd like to have the 85 1.8, but that's not gonna happen in the next couple of days (did think about renting, but decided against it), so the 55-250 is the only lens I have with the 85mm FL (136mm on 50D).

How well does it do as a portrait lens, in your opinion?

Despite the slow-ish aperture, it can be used as a pretty decent portrait lens. You can get a pretty good amount of background blur for subject isolation with the 55-250, especially at the longer focal lengths.

Just a tip from what i have learned is if you shoot all the way open, the lens is quite soft. I usually shoot at 250mm 400iso f8 1/250.