@maximuss If you compare it to the major notation programs, then it's indeed inferior, but I think it can do the job, especially when you're not into composing highly graphical contemporary music. I don't think composers who write in this style are interested in a DAW in the first place, but well, you never know.
You need to get your head around a few things to get it to work, mostly because it works different compared to the major notation programs. Sometimes it makes you tear your hair out, smash your head against the wall, throw your computer out of the window, simply because you are so used to certain key commands and ways of entering notes and dynamics, that when you do this in Cubase's score editor, you get the most weird results.
I'm pretty used to it now and it fills my needs up to a certain point, and combining this with the midi editor, will defenitely get you there.
Importing XML from Cubase to Sibelius and how good this will be depends at first on how good your score looks in Cubase, especially regarding quantization and how strict you have been in using the right channels and voicings for your instruments. For solo instruments this is easier of course, because they have only one line (except perhaps solo strings), than for keyboard instruments f.e..
Maybe write a short, simple piece, or enter an existing piece using the score editor of Cubase and export it from Cubase to Sibelius and check the results. Take the time, as even a short frase of eight bars will take some time. It feels a bit like the very first time you started entering notes into Sibelius, although this is by far less difficult. Once you're finished, export it and see what the results are and where you can improve them.
One tip: program the numeric keypad (not the 'normal numbers'!) so, that they match the numbers of Sibelius for note values, otherwise you will be moving your mouse around like crazy to select the right value. This can be done in the preferences. If you do this, Cubase will most likely give an error saying there's already another command under the chosen number, but you can overwrite this. If you want this to reverted later on, you can either change it manually back again, or reset everything to default. But please be aware that all your keycommand changes are reverted when choosing this option, also the ones you probably made in the past, so be careful.
Anyway, good luck with it. I can only say I'm really glad I took the time to learn using the score editor, albeit sometimes a frustrating journey and I still learn every time I use it. I hope the same goes for you and you will see the potential of it, as it's really much better than other people say. But of course this highly depends on what you expect from it and what your specific needs are.