A few days ago, I bought a second hand YDS-150 as well, mainly for silent practice. I have not explored the app or midi capabilities yet. I changed some settings via the keyboard, and tried some voices.
I'm a stand-in twice this week, both in a bigband, one time on tenor, and the other on alto. So far the YDS proved very useful for exploring the charts, and getting a feel for them. Especially since I can pick up the "sax" in the short work breaks (working from home), to play over some things.
I found that the T-02 and B-01 voices have very little to no vibrato (that should be adjustable indeed). One very confusing aspect for me, is that I cannot voice notes. Especially when playing along a YouTube video (e.g. Basie orchestra), and subconsciously try to match the intonation. It's weird that I voice the notes with my mouth, but absolutely nothing happens.
My initial impression, after a few days, is that it's very useful for studying (new) charts. But it's so far removed from a real sax that I probably should not rely too much on it as a practice tool.
Of course it's also a musical instrument in its own (as the recordings in this thread show). But I have not explored that part yet.