I had an Atmos receiver (Onkyo 646 I believe) for about a week while my receiver was being repaired. I can tell you this, even playing back Atmos audio with a regular 7.1 speaker configuration (nothing overhead) and the improvement in sound vs. the non-Atmos soundtrack... No overhead speakers required.
I also agree that it is important to focus on your "bed" layer speakers (traditional 5.1 or 7.1 speakers) first. These WILL product a majority of all audio even with Atmos.
With that said, I still could tell that I was missing something by not having overhead speakers. The space still felt like it was somewhat "flat" (my best way to describe it). I am confident after hearing A/B demos of 7.1.4 vs 7.1 material demos at CEDIA 2014, plus the CEDIA 2015 demos, and with my own experience with a 7.1 playing Atmos sound, that I want/need overhead speakers. Even though they may only be used 5-10% of the time, there is a noticable improvement. Heck, I was just reading something in one of my latest Sound and Vision magazines and another thing online about the top 10 technologies to look forward to in 2016 (NOT a home theater article, but list of ALL tech) and they both talked about Atmos being huge for 2016. S&V said something that I heard elsewhere before too. Dolby Atmos is to movie audio what DD5.1 was when it came out and you could send audio signals to all 5 channels and subwoofer separately. This isn't a gimmick or fad like curved TVs, this is an actual significant improvement in the audio presented.
But again, you really need to have something decent for your primary 5.x or 7.x first like Matt said above and good placement of those speakers is really important as well to proper sound imaging.
