I started off with my own plugin and ran into several issues.
First, I had to re-enter all my info everytime I restarted the app; it was unable to write out the configuration file. By playing around with some of the start-up options I as able to get the config written out, but then it couldn't connect to the Crashplan servers. I swear at one point I got everything working, but for some reason I don't have the configuration documented; I probably never actually got it perfect. I was going to try installing it in a Linux jail, but I couldn't get the jail itself to function correctly (packages couldn't be installed, permissions seemed to be incorrect).
Later, I tried just installing the plugin. Everything is almost perfect. The one issue you'll run into is when Crashplan wants to update. When 3.6.3 came along the plugin broke (I'm not sure if there's a way to keep Crashplan from updating itself). Also, there was one time when it looked like there were many zombie processes that CP had spun off. I restarted the service and things dropped back to normal. I haven't paid too close of attention to this, but it hasn't seemed to be a major issue.
You will experience a few other issues, but I consider them minor. The plugin can't track files in realtime so you'll only get backups when CP is set to verify the backup. I have this occurring every night. I'm tracking around 300 GB and my drives and CPU are busy for maybe 30 minutes. I have the stats, but I can't check right now.