I used Abeka's 2nd grade language with my son. We switched to using R&S 2 this year, and we both really like it much better!!

The expanations are simple and straight forward, and he's retaining what he's learning. I felt like Abeka's book was much more scattered...and I don't think Grant remembered much of anything from it. It was really feeling like busywork to me by the end of the year.
If Abeka's working for you though, and you like it, it won't mess up the plans at all. You're free to use whatever you want.
Michelle, just so you know, HOD reccommends that your child finish R&S 6 by the time they're finished with 8th grade. (Because of the more advanced scope and sequence of R&S, your child will be ready for high school level grammar at that point.) So, if your child hasn't done diagramming and you thought R&S 3 is where you should start, you really wouldn't have to feel behind. My 4th grader will be doing R&S 3 next year, and I feel like we're right on track.

Kathleen