I'm a professional orchestra double bass player and occasionally I perform solo repertoire which is often written for solo tuning, a tone higher than orchestral strings. From low to high, solo tuning is F#'H'EA.
Pros: the Thomastik Belcanto Solo H Double Bass string, the third string, is closest in tension and feel to an orchestra A third string. It feels quite similar under the bow and with the left hand, whereas other brands can feel tighter and may choke the instrument due to their higher tension. It is punchy and deep sounding. Another example is when performing Giovanni Bottesini's music, which is full of harmonics in the high register, and the Becanto strings really facilitate their execution.
Cons: I cannot think of any cons, I ordered the whole set of Belcantos and I would recommend them to any double bassist looking for the ideal string for solo repertoire.