I have been using Sonlight material, to some extent, since my oldest started 1st grade - so, for 3 years now. But we haven't actually bought a full core until now.
For 1st grade, I bought a used Core A instructor guide, to see how we liked it. We were using BJU Distance Learning videos for 1st grade also, so we just did the history/geography/missionary readings, and not any of the language arts or Bible parts. I discovered that the read-alouds were really fun for the kids, and that Rebekah learned the most through that style of learning (rather than textbooks or workbooks or even videos).
I debated our future plans quite a bit, considering combining Sonlight with BJU videos (which we eventually decided to not use, mostly due to worldview differences, plus they took too long and did not take advantage of the one-on-one instruction that my oldest does best with). For 2nd grade, I finally just bought many of the Sonlight Core B history books listed on their webpage, and we just read through them at our own pace. Again, this worked really well. For English, we ended up trying a variety of things - CLE (which she hated- though she loved CLE math), various workbooks (done orally mostly), Writing with Ease, Wordly Wise, Sequential Spelling, and the Sonlight Grade 2 readers, which I just bought separately and had her read as she wanted. The only thing she loved and did well with? The Sonlight readers. At this point, I began to come to terms with the fact that the Charlotte-Mason method of language arts was really the best fit for Rebekah, and that she really was learning without all the grammar drill and details. At least at this age.
For 3rd grade, we continued with our "relaxed" history method of buying the Sonlight Core C books separately, and just reading them at our own pace. For English, we started out with Rod & Staff, and made it through 3/5ths of the book before burning out and switching back to a Charlotte-Mason method. I should have known. Despite various workbooks I used for grammar, Rebekah just never really liked them, and, due to her stubborn personality, she never learned from them. But I let her read Sonlight books as much as she wanted, and her reading ability soared. Her test results this year showed her to have a 6th grade reading level (91st percentile in her grade). She finished all the Grade 3 Sonlight readers early, and I bought the Grade 4-5 readers - which she has now finished also. She's also writing more on her own now, without any prompting on my part, though that area does still need more work. Oh, and her "social studies" test scores? Her grade equivalent was 7.8 (eighth month of 7th grade), and she scored in the 99th percentile.
So, for 4th grade? I bought Sonlight Core D in its entirety. It's just simpler this way! For 2nd and 3rd grade, I'm still happy we did them the way we did, at least for history. At those grade levels, we didn't really need the full structure of the instructor guide, and I think the language arts wouldn't have been a good fit for her, especially as far as writing ability goes. Maybe it would have, I don't know. But it's worked out fine. We will use Sonlight Core D for Bible, history/geography, and language arts this coming year. And we are both very excited!
For my younger two, I plan to use our old Sonlight Core A schedule again. It's been long enough that I am looking forward to reading those books aloud again! Ryan will be in kindergarten, and Reanna is somewhere between 1st and 2nd grade, so I think Core A will be good for them. (I'll write about English plans for them later.) For the next two years, I'm not sure yet if I'll just read through the Core B and C books like I did with Rebekah, or if I'll break down and get an instructor guide. My guess right now is that I won't, since I've already been through those books once and have a good grasp of what discussions to have. But starting with Core D, we are going all out, and doing the whole thing! Sonlight for real!