Guild Wars 2 has posted up 1 million copies sold in just a few days of release.
Though most MMOs are suffering from the growing pains of a market transitioning from subscription models to a free-to-play one riddled with microtransaction and vanity items, ArenaNet's Guild Wars 2 is in the interesting position of never having to face such a dilemma. Instead, Guild Wars 2 relies on a pay-only-once model to sustain the game.
After building up plenty of hype due to the game's many innovations in the MMO realm, including its unique dynamic events and storylines, it's not much of a surprise that Guild Wars 2's done well at the numbers game. According to ArenaNet, the game sold over 1 million copies and sustained 400,000 concurrent players at its launch. No doubt early access to the game for pre-orders helped cushion the numbers quite a bit, but posting such numbers is still no small feat.
Guild Wars 2's success is refreshing, especially when other MMOs have had relatively little luck. Recently, BioWare's Star Wars: The Old Republic, despite having a strong start, has abandoned the subscription model for free-to-play (with level restrictions) after its numbers began dropping. Even Blizzard's ever popular World of Warcraft is losing traction, bleeding a million subscribers within a matter of months. One of the most recent story-based, recurring subscription MMOs, Funcom's The Secret World, is struggling to capture its market. Even vampires and zombies, usually winning formulas, aren't enough to attract players, apparently, as Funcom has revealed that The Secret World has sold only over 200,000 copies. The developer's lost something close to $35 million and has suffered a slew of layoffs.
Though Guild Wars 2 has had successes in the first few days, it'll be interesting to see the game's numbers in a few months. Will the new content that ArenaNet continues to push out be enough to capture current players' interest while drawing in new players?

The game is fun. It does not need a huge population of concurrent users for years to come to remain being fun. It just needs people who love the game. GW1 has lasted for 7 years. Even if the number of concurrent users dropped in a year, so what. As long as people are playing, guilds are active and it is fun, most users will be happy.
Then like they did with GW1, there will be expansions. That will increase their revenue and bring back some interested in new content and maybe even attract new players.
So again I ask, how do you measure the success of GW2 given its business model? Box Sales? Then yes, it has achieved its objective.