So far no sales numbers are known. But there are other ways of measuring SWTOR stuff. There's server numbers, and tools like Xfire and Raptr that measure player activity of gamers using them.

So, how is SWTOR doing compared to other MMO's?

Ok, here it goes:

Number of servers
At the moment, SWTOR has 140-150 servers. On average most MMO servers (or MMO server clusters) can have 15-20k players linked to it at max.
Rift had like near 100 servers at launch, people estimated Rift's player population around 800k, but Rift has 1 of the smallest worlds and lowest player caps (no more than 2000 players can be logged in, while other MMO's could reach heights of 4000 to 6000 players logged in on a server). Aion had like 33 servers for 900k US/EU players at its launch, and AoC around 50 servers for 500-700k players.

Xfire and Raptr
At the moment, the number of hours of SWTOR players that are clocked edges still closer to the number of hours clocked in total for WoW.
In Xfire, SWTOR at the moment takes the 5th-6th spot.
When Rift launched, it had 30k hours in Xfire as a daily peak, and 5k Rift players (now around 400 Rift players in Xfire and 10-15% players in Raptr and Steam currently compared to March when Rift launched). So far, SWTOR registered 73k hours and 8.5k players in Xfire. EVE Online, which hovers consistently around 350k subs/players registers 1.3k players in XFire.

I think it isn't that far off to think that SWTOR at the moment has at the least 1.5 million players, which makes it the biggest MMO launch so far. The rest has stayed below 1 million players at their launch.
We'll see how player activity continues after launch.