N900 from day 1 wasn't super fast. It was smooth with nothing installed but after a while, animations became visibly choppy and even loading up Settings took a few seconds. I felt the need to tweak & overclock within few weeks of getting the N900. My N900 is visibly quicker compared to a stock N900. (I run mine at 1.1GHz) I've never had stability issues nor any battery life issues.

Also, the fact that N900 drops frames even while recording video, taking a long time installing/uninstalling apps is pretty bad. The new SGS2 takes around 1-3 seconds literally to install or uninstall any apps. But yeah, I sold my SGS2 and still kept the N900 due to its hw keyboard, multitasking and MicroB. N900 rocks.