I just set it up last week to push data to Graphite. It took a little bit of time to understand how to configure it and the docs have conflicting information in some places. Also, you will need to build it yourself or get a PPA if you're on Ubuntu 12.04 and want to use it with Graphite since the version that Ubuntu ships with doesn't support graphite.

I haven't tried to write plugins for it but it comes with a lot out of the box and it's working well.

collectd is great because takes very few resources (cpu/memory) to collect statistics very rapidly and you can log your results however you wish (back to graph collectors, straight to CSV for later processing, out to custom processes or network protocols for other services to consume).

If you want to be lazy and not set up a complete graphing infrastructure, just run collectd, have it automatically log all your statistics, and use it with the bundled https://collectd.org/wiki/index.php/Collectd-web package to view your history when needed.

I use collectd and have found it to work rather well. The documentation could certainly use some work, but once you get figure things out it's pretty easy to configure. CPU usage is almost non-existent for the amount of data monitored. It's also pretty easy to write a custom plugin to collect whatever custom metrics you want. I use a script to expose the current values out as http/json for integration with Circonus for monitoring/alerts of key values. You can use whatever graphing tools you'd like, things are stored in rrd format.

Recently I've been implementing salt stack, and collectd is really easy to automate config of. I've got salt fully configuring collectd, and then using the circonus api to setup monitoring rules automatically per applied states. It's a beautiful thing.