I've been a Finale user since about '92, and I've grudgingly made various upgrades to currently using the 2005 version. It seems like they've been doing "major" upgrades that they charge $100 for almost every year! I wouldn't call myself a power user; I'm sure I'm just scratching the surface of what the program can do.
I bought Sibelius a couple years ago and tried it a couple of times. It is much simpler to get into than Finale, but my Finale habits made the switch difficult, so I went back. I encourage my students to start on Sibelius.
I like being able to hear playback (being a lame pianist, it's a godsend!), and being able to edit more easily.
But, here's a possibly contentious statement: I think the downside of the whole notation software thing is that it has made it too easy for really lame, half-hearted writers to print out neat and tidy versions of shitty music. With pen and paper the writer had to be committed enough to the project to write out all the parts (or rich enough to get parts done by a copyist). I think that hurdle saved musicians from having to read a whole lot of crap.
Sure, with computers and the internet any idiot can have a webpage and a blog and spew out endless streams of nonsense, but I don't have to sit down with him, read through his babbling, and then try to say something diplomatic. "Yeah, man. That was really...uh...something. And the copywork looks so neat!"
Now, I'm sure that the musical output of the present company is certainly worthy of seeing daylight...