As you walk around the island you see the red bus trumping up and down. It makes everything look easy and the distances doable. It was also recommended to me by a friend who visited Malta previously. The bus is on both of the islands and I tried it in both. This is what I thought in details:
The Hop on and Hop off Bus in Malta
Bus costs 15 euros per person (usually 20 according to the driver)
The distances of Malta are vast and it takes a while to get from one end to the other. We got on the bus from the last ending point Rotunda because the bus from the Ferry took us there. Then we made our way to point one in Bugibba all the way to point 22, which took two hours. The route map with numbers below or could be accessed from the official website. We could have gotten off, but there was not much to see in terms of the architecture in that route we took. We passed by few quiet beaches though.
The bus goes over the edge of the island so there are a lot of sea fronts to enjoy. The upper deck was nice, but then it got really windy. Being on the bus gives you an over view of the island. The downside was the information points on the bus didn't work, so it was like riding any bus without knowing any inside information about the place. Also, the famous places are accessible by public buses and only costs you 1.50 euros per person. Due to the big distances of Malta, I felt like the bus was taking much longer time than it needed. It also stopped at a number of hotels and sea fronts rather than historic and natural sights. Without being on it, I wouldn't have known how unnecessary this bus was. I would not recommend it for Malta.
In case you want to book the Malta Hop-On Hop-Off Bus Tour
The Hop on an Hop off Bus in Gozo
Bus costs 10 euros per person (usually 18 according to the tourism office)
Since Gozo is much smaller, getting the Hop on and off bus was a very different experience. Plus, the information deck was functional, so it was nice knowing more about the place while passing it. With the Gozo bus, we actually hopped off a few times to enjoy the place for 45 minutes at a time and then headed to the next stop. The only downside was we took the bus slightly later and didn't enjoy it to its full potential. I guess Gozo as well could have been done with just the normal public transport bus, but the hop on and off came quicker and followed a nice route. I think it was worth it. My friend E and I really enjoyed our time on it!