I am doing Omnibus I over 6 months with a 6th grader, and beginning Omnibus II in August or September as she begins 7th grade. I have spent time looking at all of books to be read, and deciding on the ones that are keepers. Almost all of them are (95%). My daughter LOVES to read and do analysis so that's the main reason why it works for us. I also read a lot of the books out loud to her and discuss as we go along to make sure she is grasping it. (She does, but my reading it aloud to her is key.)
Here are some pointers I have found helpful:
At www.Sparknotes.com there is a section called "No-Fear Shakespeare". The entire text of all of Shakespeare's works are included in the original language (free) with a modern translation right beside it. This works out wonderfully. We're not losing Shakespeare's words, but we have a good translation right next to each passage for those tough to understand parts.
For Beowulf we'll be using Beowulf: A New Verse Translation.
For R.C. Sproul's books Chosen by God and Holiness of God, I had to highlight the important parts because his writing style is such that he repeats the same things ten different ways. It was super annoying. We're deleting Holiness of God, and using a chapter of Grudem's Systematic Theology to make of for the teaching in this area....not official Omnibus for that book.
I find everything else to be doable, but at this age it's a larger parent commitment. If you feel led to do it, do it, and make whatever tweaks you feel are necessary for your child. The ideological lessons to be learned here are priceless, not to mention the in depth study of great literature.
Another thing: We skipped all of the studies of books in the Bible. This is because we study the Bible in-depth using other curriculum. This allows us a little more time to spend on the meat of Omnibus. I am not discounting the Bible study at all...we do it, but cutting it from Omnibus was helpful for us. Another thing to keep in mind is that Omnibus teaches from a position of dominionism. This is not a problem for me, although I do not subscribe to dominionism personally, because most of that occurs within their studies of Revelation and Daniel, and is so lightly sprinkled in elsewhere that it is not hardline, or what anyone would call "unorthodox". It would never deter me from using the program.
Overall, Omnibus is solid, and a must have in my opinion. The key is spending some serious time poring over the material in advance and discovering what books you want to cover and how you are going to tackle them. More parent involvement is the key for younger grades in my opinion. I hope this helps you, and God bless your endeavor!!