The first rule of Reaper's pre-release forum is that we don't talk about the pre-release forum, but teak they've been working hard on getting this right. I believe it was meant to be in the 5.0 release, but they didn't want to just tack on some half-assed notation view, and it became a distraction from a lot of the other things they were trying to do, so they shelved it and have been hacking away at getting it to be actually useful and intuitive. There has been a lot of back and forth and they have been (as usual) very responsive to user feedback. The next step is going to be incorporating MusicXML import/export to go between different notation editors, but I think that's going to wait til after they get 5.2 out.
As for volume, notation doesn't usually show any kind of absolute velocity information. You'll have general ranges - forte, piano - and things like crescendos can be notated. I know this thing allows you to notate those, but I'm not sure how (or if) they actually affect the velocity of individual events. I'd imagine there's got to be a way to set velocity from notation view, but I haven't actually tried it.
Course, this feature does me no good whatsoever, but people are pretty excited about it.