hi
the NTL news server is ok to 'practice' on, but you may end up getting frustrated when you get a bit more into newsgroups.
The download 'limit' is subject to the same 'fair use' policy as your overall connection.
In my opinion the main 'problems' compared to paid for servers are that many busy binary groups, especially media ones, are not carried, retention times can vary depending on how busy the groups are, and the completion of multi part binaries can sometimes be iffy.
However, it is effectively free, so I don't see why you should't give it a go.