It is a fantastic PSU, but it is a lot more expensive.  Corsairs use Seasonic internals as well, so they are very similar in terms of quality.  If you are looking for a modular PSU then the Seasonic is a great option (Corsair makes a modular model also, for roughly the same price).

 

I have both and they are both fantastic. The Seasonic is excellent for HTPCs (yeah, even if you do not need 650W) as it will run fanless if output is less 20-30%, otherwise the fan kicks in as it starts to heat up. However, it's not a single rail and thus "not good" for a server like a 20 drive unRAID box, IMHO. Get the Corsair 750HX, it is modular and same build quality. As Rajahal mentioned both are Seasonic internals.

 

There is also a SeaSonic X750, remember that the PSU will go fanless until a certain amount of output power threshold is reached, thus the more you have the more likely it will go fanless all the time. Important, if less noise is what you are after.

 

So, kind of Corsair 750HX is the SeaSonic X750, same for the 650 models, and all they are modular. Corais is $20-30 less expensive in general. On the contrary Corsair TX is not modular and a different PSU type and price class.

 

The Seasonic is excellent for HTPCs (yeah, even if you do not need 650W) as it will run fanless if output is less 20-30%, otherwise the fan kicks in as it starts to heat up.

 

Seasonic even makes totally fanless PSUs!  400-460W should be plenty for most HTPCs.

http://www.seasonicusa.com/NEW_X-series_Fanless.htm

The Seasonic is excellent for HTPCs (yeah, even if you do not need 650W) as it will run fanless if output is less 20-30%, otherwise the fan kicks in as it starts to heat up.

 

Seasonic even makes totally fanless PSUs!  400-460W should be plenty for most HTPCs.

http://www.seasonicusa.com/NEW_X-series_Fanless.htm

 

Yeah, the X650 is also "totally" fanless as the fan is there only as a reserver, should the computer overheat; without it it will just smoke up. But the other ones are cheaper... and also good!