Depending on the width of the lane, the speeds and number of buses, and other local factors, the safety and popularity of this arrangement vary.
[1] Guidance produced for Cycling England endorses bus lanes because they provide cyclists with a "direct and barrier-free route into town centres" while avoiding complications related to shared-use footways.
[2] A French survey found that 42% of cyclists were "enthusiasts" for shared bus-bike lanes, versus 33% who had mixed opinions, and 27% who opposed them.
According to the Sustainable Safety guidelines they would violate the principle of homogeneity and put road users of very different masses and speed behaviour into the same lane, which is generally discouraged.
[11] Incidentally, the TRL reports that cyclists and bus drivers tend to have low opinions of one another.