I tried Calvert with my older son (3rd grade materials) for a few months this school year as a first-time delving into packaged curriculum for us; it was free with enrollment in a local online school.
My opinions --
Pros: The daily lesson plans are SUPER great if you're at all disorganized or unsure of how best to get all the subjects tackled on a regular basis. There's a list at the top of which books are to be used that day and which pages, and there's actual verbage you can use when presenting the materials. There's an online "Checkpoints" thing the kids can do each day where there are 3-4 multiple choice questions on each subject which sort of skim the surface of the info they learned that day -- not a thorough review, just a quickie thing, but if you want to keep track of what lessons you've completed it is good for that as well as when you log in and do a checkpoint, the system counts your student as "attending" that day if that is something you're interested in or required to track. There are some fun online Math games connected with many of the lessons, and the online spelling program is also fun (which you can do in addition to or instead of the book/paper version).
Cons: Social Studies text (3rd grade) is ULTRA boring. IMHO it is a "weird" subject anyway - I think pure History is more interesting myself, and so do my kids. Read the book, look at a few pictures, write down new vocabulary words. The Science text tries hard with a lot of nice photographs and there are occasional online links for more exploration, but in general I thought the book was pretty lame. The hands-on stuff is pretty much non-existent, and it made my thrilled-about-science kid say he "hates Science" after doing a few months of "Science" using the textbook. The Composition work was quite challenging for my reluctant/inexperienced writer -- they do start very simple with having the parent write out sentence phrases for them to copy into their compositions, but it seemed quick to me that they jump to having the kids write 3-4 paragraphs all on their own. I'm sure a kid who enjoys the act of writing would be just fine; my guy *loathes* it (he's happy to narrate things and comes up with stories on his own, but the physical act of writing really blocks his creativity).
Good luck with your decision!