I've used 2A a bit with DD7. It's simple, yes, but I've done a few problems on weekends to keep math fun for her. And she loves the Guide Book, finished it the first day we got it. I've mostly had her do the challenge problems, and they're interesting enough to keep her engaged, while not hard enough to get her frustrated. But my philosophy has always been that there's benefit in doing problems somewhat below their level, to cement concepts and boost their self-esteem.
Our situation is different, though. I'm doing this mostly because I don't like the math they're doing in school, it's pretty rudimentary and repetitive, and she's bored out of her skull. So I've been using this to reinforce to her that math can be like puzzles.
We have 3A as well, and do the opposite there, the regular problems are at about the right level but I'm not trying the challenge problems because she gets easily frustrated (she has ADHD) and I want her to keep her enjoyment of the program...Saving those for this summer.