I've used librouteros (→wiki, →thread), the library I wrote last week, to implement a plugin called “RouterOS” for collectd, a performance measurement daemon. Because my focus is towards point-to-point wireless LAN connections, that is exactly what is currently supported. Of course, other statistics may be added in the future.
Currently collected interface statistics are:
- Packets sent / received
- Bytes sent / received
- Errors
- Dropped packets
- RX / TX bitrate
- Bytes sent / received
- RX / TX signal strength
- Signal to noise ratio
- RX / TX CCQ

The code is currently living in the ff/routeros branch of my collectd Git repository. If you want to give the plugin a try, check out the repository from git://git.verplant.org/collectd.git.
Of course, feedback is very welcome :)
Regards,
—octo