The main advantage with Usenet over BitTorrent is that with Usenet, when you download, you are downloading only. This potentially makes you safer as you are not sharing out anything over the duration of the download, let alone after it completes. The second advantage is that it does not clog up your uplink, so if you have a limited uplink (e.g. <512kB), you can still comfortable browse the web, use SkyPE, VoIP, etc. It is up to you whether you want to share files out on Usenet. With most providers, the download runs as fast as your Internet connection will handle.
For the Usenet provider, I would suggest giving Giganews a try. You can cancel your subscription within 14 days and they will not charge you, at least going by what I can tell on their website. All good news providers do charge a premium. If you come across a free Usenet server, it will either carry no are very few binary groups.
To download files, you need a Usenet client capable of downloading binaries. Basically, what you do is select the NZB file and it will connect to your usenet provider to download the binary file. The NZB file is basically headers for the binary, so the Usenet client knows what to fetch to get the binary, much like a Torrent file.
For the Usenet reader, I would suggest having a look around to see if you find a client you like. Personally, my favourite is Newsrover. This application is not free (costs ~$30), but it has an excellent search engine which I haven’t come across anything alike for Usenet.
If you would just like to search/browse Usenet for NZB files to get an idea of what’s available, have a look at binsearch.info