I bought my second hand halfbike on ebay, and I absolutely love it. I use it for commute to work 9km one way - so 18km of halfbiking per day!
First reason why I love halfbike: I can combine the commute and have a workout at the same time - gives me more free time for my other training, chilling, and my friends and family.
Second reason why I love it: it is amazing workout. I am a long distance runner (ran mountain marathon) and road cyclist (cycled 230km in one day). But my legs and core were shattered after first 2 days of halfbiking. I can't wait to see the impact of long term halfbiking on my fitness performance.
Third reason why I like halfbike: it is such a good fun. I am commuting to work and have feeling that I am on spinning class. I can use my headphones which I never have while I am cycling - for safety reasons.
For me halfbike was super easy to learn, it took me around 10 minutes to feel comfortable while riding it (I am 27 and was training martial arts, running, cycling since I was 10). My 3 friends just stood on the halfbike and rode off from first attempt. Highly recommended product.