My two most active buy-on-sight authors are Lois McMaster Bujold and Diane Duane.
There is a Lois McMaster Bujold community on LJ:

Flagged for the attention of new reader

Anne McCaffrey and Mercedes Lackey are no longer buy on sight authors, due to one or more of the following factors:
- decline in quality of work
- me growing up
John M. Ford would be a buy on sight author, except that I seem to own many books that he's written and haven't run into one of his that I don't have in years, and that I'm not seeing much new material from him.
Robin McKinley had been a buy on sight author, but had been inactive for a while; Patricia McKillip is becoming one. C.J. Cherryh has been one for a while, but her writing is developing more, for me, into read-at-library-first series, and buy-on-sight series.
I will buy other books, but I have to get to know them first: someone must recommend them to me, or I must have read them before, or I must be sold on them by cover art grabbing my eye, and read enough of it to know that I'll want to read the rest and re-read it.
Re-readability is the real test of whether or not I think I'll want to buy a book. Books are not disposable, to me. I read fast enough that I zoom through books in a few hours (if I have the time) to a few days (if I'm somewhat busy) or up to two weeks (if I'm absolutely jammed for time). If I buy a book, I'll want to keep it, and if I keep it, I'll either want it for reference later, or I'll want to read it more than once again.