Before installing denyhosts, I used to have thousands (yes, thats right) of attempts with usernames starting from A to Z and other dictionary words. None was successful!
Then I installed denyhosts a few months ago and as of now I have exactly 160 blacklisted IPs. Definitely denyhosts is a must to keep hackers away.
It can not only blacklist IP addresses service wise, it can also block the IPs entirely (all services).