I always like the Rosemary Edgehill 12 treasures books, but more for the academic nerdi-ism than the portal stuff. I do like Wen Spencer's Tinker series--I think it does the portal thing well. And while I liked the Andre Norton Witch World stuff, I did not like the crossover spill of worlds or the portal part--I like the witch world part of the books. I also like the Amber series, which is a version of the portal story.

It seems in the ones that I like, that there was a reason for the portal or that there was story that dealt with the portal in a way that it just wasn't the convenient macguffin that allows a passage but other than that has no life ramifications for anybody.