Yellowbus

First the good news; they have 4 decent routes to get a good perspective on the city, and the 48hr ticket is reasonably priced and allows you to also use the public transportation (other than Subway).
Now for the rest of the story. The buses are in good condition except for two key elements; one, many of the headset jacks did not work or had loud static (on one bus, only one out four headset jacks nearest to me worked properly), and the seats were wet. I don’t mean they were visibly wet. The seats have a drain hole, and water had seeped into the foam seat around the drain hole and when you sit on the dry-​looking seat, the water squeezes back out of the foam and soaks your butt. Some of the seats were cracked and these were even worse.
The people operating the bus were all rude. The person checking tickets on the sidewalk and the bus drivers were not polite or friendly in any way. They all appear to hate their jobs. At one point we tried to board the bus at the starting point and were just told No!, and were motioned brusquely to go away. It took about ten minutes to get the sidewalk attendant to tell us what was going on. Apparently there was something wrong with the bus or the driver (the attendant wasn’t clear) and the police were removing the bus from operation! We waited over 45 minutes to get the next bus, even though we were told initially it would be 15 min.
The buses were operated in a very jerky manner that made it difficult to take photos. At one particularly rough stretch, my wife went to see why the driver was so driving so erratically and he was texting on his cell phone! She told him to stop using his cell phone and drive.
My advice is to try the red bus.

It is a great idea to get your bearings around Lisbon, when you are first time in the city. The 48 hour ticket allowed us to take all of the yellow bus tours, except the tram one. The ticket also covers local transportation, like buses and trams. You can just hop on and off wherever you want. They have a pretty interesting guide throughout the tour and you get free set of headphones when you board. Highly recommended!

Great way to visit all the sites and get a good over view of Lisbon. We purchased a 48 hour ticket which allowed us to use the yellow busses, trams, funicular and go up a lift to a view point.
The bus is open top and you can hop on and hop off. However, we ended up riding most of the way around first so we had a good idea of the city.
The commentary on the bus is also informative.
Trams are also great — very traditional and fun.
Would recommend but once you have your bearings it’s a lot cheaper to use the metro /​pay for trams, busses etc directly

Seja de eléctrico ou de autocarro, os circuitos por Lisboa fazem-​me sentir um orgulho imenso por uma cidade tao extraordinária como a que temos e a cada minuto teremos de valorizar, melhorar e integrar com a qualidade de vida das pessoas que cá vivem.
Os preços não são acessíveis, mas é um prazer aqueles autocarros de dois lugares, descapotados e que nos deixam o cabelo ao vento ao sabor das ondas do Tejo, das birras dos Velhos do Restelo, da regeneração chamada Parque das Nações, da «linhagem» de Cascais ou do byronismo de Sintra. Lindo!!!