My daughter started the trombone in September aged nine and a half after having insisted she wanted to learn for almost two years. (She already plays the cello so she could read bass clef). Like you I was completely mystified by it and could not understand why she wanted to learn it. (I have tried a few times and am not much good at getting a note out of it).
Six months on she can play an octave + two notes and is proud of her rendition of "Old McDonald Had a Farm."
She has one valve (key) for seventh position on her trombone as her arms are too short. For the first two slide position she learnt you line the slide up with other parts of the trombone (so in fact it is not quite as mysterious as it seems).
In conclusion, intontation is sometimes a bit iffy but no worse than the cello (and in fact probably better). Progress has been much faster than the cello and even from the beginning the sound was not disagreeable (the way beginner strings can be).