We have used this kind of bus service in other countries but the one in Malta sucks..big time!. 17 euros for a 1 day pass, but the Red and Blue service doesn't start until 10:00 and then finished around 16:45.
The Green shuttle service however starts at 8:30 ish and stops at 15:15 so if your staying in Sliema you'll have to make your own may back. Surely it would be common sense for the shuttle service to run longer than the sight seeing service so people can arrive and then get home.
The other issue is the poor signage at the bus stops. At the main bus terminal in Valletta where the Blue bus stopped there was no sign, this was also the case for Valletta Castille and the San Antonio Gardens. The buses seems to use the normal local bus stops, but unless you were dropped off there you would never know where to hop back on.
My advice would be to use the local bus network its only 1.5 euro for the day and you can get on and off when and where you like and as often as you like. Also the local bus timetable is very frequent and easy to follow, and so long as you can see a bus stop you know your going to get picked up.
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