Some hosting plans say they give a unique IP address for your site, how important is this, or do all sites get a unique IP address from all hosts anyways?

You may need a dedicated IP for:
-SSL certificate
-Anonymous FTP

It's just that they map an IP address to the server and assign it to you. The above reasons are why it would be needed, otherwise it really doesn't matter much (for the most part - this alone could be a thread in itself).

It is handy for some things, like redirection scripts, etc., but otherwise I woldn't base my hosting choice on it. It would be nice to know if they'd give you one if you *needed* it.

They don't express themselves very clearly do they.

Actually every IP is unique.

But sometimes one (unique) IP is shared by several web sites.

From other hosting providers you can get your very own IP, you don't have to share it with others. That's what they mean.

If you want to test your siteperformance on the hosts first without transferring the domain, it could be useful. If the site host isn`t good, just claim your 30 day guarantee (wich is common) and you`re free to test another within the day.