2,5 but closer to 2 (a 6-6.5/10). Almost all the stories are good(ish), but none memorable enough to remember 3 days later.
Plenty of great names on the cover, no great story inside - not even King's or Martin's; it is obvious the editor chose the names with selling power, not the texts, so we get a very forgettable collection.
Also, the anthology is too big for its own good - about 30 stories on just 2 subjects (parallel worlds, portals), so it gets rather boring to read after half.
Not really a waste of time, but not a book to recommend, either. ( )

Wide variety and variation in (short) stories but overall good. ( )

There are a lot of big names in this collection about parallel universes and portal worlds. On the whole it was a bit underwhelming though, with many stories lacking originality.

Individual Story Ratings:

Foreword — Lev Grossman (no rating)
Introduction — John Joseph Adams (no rating)
"Moon Six" — Stephen Baxter (***)
"A Brief Guide to Other Histories" — Paul McAuley (***)
"Crystal Halloway and the Forgotten Passage" — Seanan McGuire (***)
"An Empty House With Many Doors" — Michael Swanwick (**)
"Twenty-Two Centimeters" — Gregory Benford (**)
"Ana’s Tag" — William Alexander (***)
"Nothing Personal" — Pat Cadigan (**)
"The Rose Wall" — Joyce Carol Oates (**)
"The Thirteen Texts of Arthyria" — John R. Fultz (****)
"Ruminations in an Alien Tongue" — Vandana Singh (*)
"Ten Sigmas" — Paul Melko (***)
"Magic for Beginners" — Kelly Link (***)
"[A Ghost Samba]" — Ian McDonald (****)
"The Cristobal Effect" — Simon McCaffery (***)
"Beyond Porch and Portal" — E. Catherine Tobler (***)
"Signal to Noise" — Alastair Reynolds ( ***)
"Porridge on Islac" — Ursula K. Le Guin (**)
"Mrs. Todd’s Shortcut" — Stephen King (***)
"The Ontological Factor" — David Barr Kirtley (***)
"Dear Annabehls" — Mercurio D. Rivera (**)
"The Goat Variations" — Jeff VanderMeer (***)
"The Lonely Songs of Laren Door" — George R. R. Martin (****)
"Of Swords and Horses" — Carrie Vaughn (***)
"Impossible Dreams" — Tim Pratt (*****)
"Like Minds" — Robert Reed (***)
"The City of Blind Delight" — Catherynne M. Valente (***)
"Flower, Mercy, Needle, Chain" — Yoon Ha Lee (****)
"Angles" — Orson Scott Card (****)
"The Magician and the Maid and Other Stories" — Christie Yant (***)
"Trips" — Robert Silverberg (****)
For Further Reading — Ross Lockhart (no rating) ( )

This is a nicely balanced selection of short stories about traveling to and among parallel worlds, with a good mix of classic and new writers. I preferred the science fiction (multiple universe theory) stories to the fantasy (portal to magical worlds) ones, but all of the stories were entertaining. Standouts were: "Magic for Beginners" by Kelly Link; "Porridge on Islac" by Ursula K. Le Guin; "Mrs. Todd's Shortcut" by Stephen King; "The Goat Variations" by Jeff VanderMeer; and one that absolutely blew my mind: "[a ghost samba]" by Ian McDonald. ( )

An eclectic mix of fantasy and Scifi, some enjoyable and some really not that good... ( )