I use Netflix.
Not all Blockbusters participate in the online system, meaning you CAN'T return your movies there and you CAN'T exchange them. My local Blockbuster doesn't participate and before that came out they didn't participate in the 'End Of Late Fees" policy either.
As for shipping speeds, I've had amazingly fast results, both in getting my movies and getting notification that they've been returned.
I have noticed, however, on rarer titles not every location has the DVD. In that case, they're sometimes shipped from other locations and mailed back to those locations. A few anime titles came from Pittsburgh while all of mine usually go to Gaithersburg, MD.
Their "Watch Movies Now" feature doesn't interest me in the least because I'm SURE it will require Windows and/or have Digital Rights Mutilation (DRM) encumbering it.