While ago, I saw a comment on Reddit which said 'why Sagittarius?'. At that time, I've just thought 'why not?' and 'then why Racket or other implementations?'. Now I still don't have that much reason but at least there should be a good reason to be chosen! So trying to convince people who are looking for a good reason to switch or start trying new Scheme implementation.
N reasons why you should use Sagittarius.
Sagittarius is supported on number of platforms, such as Windows, Cygwin, Linux, QNX, FreeBSD*1 and Mac OS X. A lot of implementations don't have native Windows binary support. So if you need to run Scheme on Windows, then this is one of your choice!
2. Architecture independent FFI support
Sagittarius is using libffi for its FFI so most of platforms can use FFI.
3. MIT license
GPL or LGPL is not suitable for a lot of situation (say Racket is LGPL).
4. Short term release cycle
Currently Sagittarius is being released once a month so if you find a bug, you would get the fixed version next month.
5. Both R6RS and R7RS are supported
As far as I know, Sagittarius is the only implementation which supports both R6RS and R7RS (currently).
6. Reader macro and replaceable reader
If you don't like S-expression but like Scheme or want to use some of libraries written in Scheme (or Sagittarius' builtin libraries), this would be your choice!
You will probably find more reasons once you start using it. So why don't you give it a shot?
*1: Current FreeBSD ports has Boehm GC 7.1 however this version doesn't work on it. 7.2 or later needs to be installed manually.