I rode one for five days a couple of years ago, helping to get it from John O'Groats to Land's End for Cancer REsearch UK. We had a lot of fun, but yes, it was hard work. Those little short cranks and the low gear mean keeping it moving at any speed requires you to spin like crazy, and getting it up hills takes some serious grunt.
When three of us pulled up on it to the HIlton Hotel in Leeds where someone had blagged us free hotel rooms for the night, we were really hoping to hand it over the valet parking. But alas he'd clocked off for the night when we got there and we had to pedal it up the ramp into the multi-storey car park ourselves.