I liked it, for the most part. Your writing flows smoothly and especially the beginning was almost too depressing to go on. You grabbed at my heartstrings and yanked fiercely. The sex scenes, in particular that phone call, were fine. In general, I liked the fine line you walked between building your characters and building sexual tension. Also, some genuinely funny dialogue.
Too bad that fine line gets tossed out the window once Cass and Alex let go and the story wades deeply into strokefest territory. The climax you've built towards felt off, and I'm not quite sure why. Maybe Cass' transformation from "OMG, wrong" to Alex' cock slut came off just a little too drastic for my taste. Maybe because the first, explosive lovemaking was just a Director's Cut of what Alex did with Meredith. But that's just my two cents.
Also, do yourself the favor and try to find an editor. The story as a whole was fine, but there were enough dropped words, slips in time and extraneous commas to*distract me.
I gave it a 4, for the reasons summed up above. I would like to see more of your work, there is potential, you'll need just a little fine tuning. If you can't find an editor, hit me up and I'll go over your stuff. Even if it is not my strongest suit, a second pair of eyes usually helps to spot the things the spell checker ignores.