I got a QY20, the sounds on all the QY's are very basic but for the size and portability they make a great little MIDI sequencer. Most people refer to them as musical sketch pads and I guess that's how I use mine.
The portability is the main advantage of the QY's, I get about 4-5 hours out of the QY20 running on batteries, but I don't know how the QY70 and QY100 compare in terms of battery life.