I highly recommend Helena's place!
First and foremost, the location is ideal if you want to be in the middle of town and within walking distance to stores, restaurants and nightlife as we did. We truly felt like we were in the heart of it all. There is a ton of street activity at night, but the apartment itself is absolutely quiet once inside -- so you get the get the best of both worlds!
The layout and size of the apartment was ideal for our situation (one couple and two single adults) such that we didn't feel like we were on top of each other. The upstairs bedroom does get quite bright in the morning, so I would recommend a sleep mask if you are a light sleeper and want to sleep in. (The downstairs room is considerably darker). Each bedroom has its own dedicated -- and very modern -- bathroom. Shower pressure was surprisingly good. Kitchen had everything we need (granted, we didn't actually cook). Internet speed was top-notch, and we got a lot of use out of the smart TV.
Although we didn't use the roof terrace much due to the daytime heat, it was a great feature around sunset. Great views over the rooftops.
Speaking of heat, the A/C was life-saving. Quiet, very effective and available in both bedrooms and the living room.
Helena and Niko were great communicators and extremely responsive, not only during the booking process but throughout our stay as well.
I would definitely recommend Helena's apartment and would happily stay there again on my next trip to Mykonos!