I use CEP with a Bose and find it very good. The Bose is great at removing low freq noise, but I would rate it as poor for hi freq. If you fly piston the Bose is fantastic. In the Puma types I fly there is a great deal of hi freq noise and have been advised after company tests not to use the Bose. Some others have even gone back to standard Peltors and leave the Bose at home!
Anyway I rate the CEP system very highly for hi freq noise reduction, combined with the Bose for the low freq, I believe this combination the best I have tried by a significant margin. Flying over 7hrs in a day used to give me a nasty headache, with any headset, not any more.
Downsides: Extra wires can be a pain, the earplugs can be tricky to fit properly.
Overall well worth the extra hassle, I did a short flight the other day with just the Bose and couldn’t believe how noisy it was.
I fitted the CEP myself and can give anyone a few pointers, but after the experience would recommend paying someone else for an easier live.
Hope this helps.