Yes, it is. You should design your login system assuming that the username is public, of course (otherwise you would facilitate attacks targetet at a specific user), but for an attack on the site itself that needs just any one account, separate error messages reduce the complexity by several orders of magnitude.
Assume 8 characters out of A-Za-z0-9 for the username and 8 character out of 96 printable chars for the password. With a single error message, the search space is 62^8 times 96^8, with separate error messages it is 62^8+n times 96^8, with n being the number of users. This is far, far smaller if the number of users is noticably less then 62^8.
This doesn't work very well if your password reset screen gives errors if the user doesn't exist. Or you can look up users by a given url. (deviantArt.com gives each user a subdomain that is the same as their username, easy lookup)
Assume 8 characters out of A-Za-z0-9 for the username and 8 character out of 96 printable chars for the password. With a single error message, the search space is 62^8 times 96^8, with separate error messages it is 62^8+n times 96^8, with n being the number of users. This is far, far smaller if the number of users is noticably less then 62^8.
[edit: stupid comment parser ate math symbols]