I love it. I have a 5 channel amp for the main 5, but then use the internal amps for a full 11.2.4 and Im in heaven. I came from an older marantz 7002 and it sounds better than that one did. The atmos upmixing is pretty good. I wouldnt call it amazing, but its pretty cool. Full blown atmos sources are freakin insanely awesome tho. The thing I really love about atmos is the surround sound just kinda comes from everywhere now. It doesnt sound like theres a speaker behind you, it just sound like its all over the place.
The funny thing is, I never wanted atmos. I avoided it thinking it was a fad. I wanted a new receiver for one reason and one reason only. I needed volume control on screen lol. Once I had the 3400 I thought, f it, lets do atmos since it supports it. Then I was sad when it didnt. Then I got the 4400 and was happy again!
But heres my thought, you have that big ole amp you just bought. So I dont think you need the 4400 unless you plan to do a full atmos setup. The 3400 that I ran for a couple weeks sounded the same to me with my external amp.