Hello Ged.
For examples of what Scribus can do, your best first stop should probably be here:
http://wiki.scribus.net/canvas/Success_stories There are pages of testimonies going back over ten years showing what some people have created. And this is probably just the tip of the iceberg.
As far as I know Scribus can't import PageMaker files but if you could export from PM to another format it might be possible to not have to start again totally from scratch.
The main function of Scribus is to make PDFs ready for printing and it has plenty of professional output options.
For formatting functions it really depends on what you expect. Scribus has all kinds of typographical functions but they might work differently to what you're used to. Not necessarily worse - and maybe sometimes better - but differently. All of the functions you mentioned are supported - amongst many others - but they might be handled in a different way. Whether that is "as good as" would be for you to decide.
Scribus doesn't really handle HTML - as such - and can't create web pages (if that was what you were asking). It creates XML which could - theoretically - be converted to HTML but there's no native HTML support.
One thing that people coming from other products have said is that Scribus works differently from what they were used to but once they get used to the different way they generally have no problems.
As Scribus is free to download, why not just give it a try and see what you think? As long as you download from the official site
http://wiki.scribus.net/canvas/Download you can be sure there will be no nasty surprises "along for the ride".