I think that most people should find that they have no problem with logging in. However, I am sure that some people may have difficulty.
The most likely cause of a login problem is a corrupted password. This is most likely to happen when non English characters are used in the password. However it could occur for any of a number of reasons. If this happens to you, remember to click the link which says "I forgot my password". You should probably be able to take care of the problem that way.
If you still have a problem logging in, by all means drop me some email.