Apps very often Force-Close now.
the OS overall is much slower.
I am not sure why people are indicating that ICS 4.0.3 is so much better than gingerbread (for the skyrocket i727). I wish I didn't upgrade and waited for the ATT release, but no turning back now...
Slower? My phone has been noticeably faster and more responsive with ICS (leak or custom rom). I've even had quadrant scores reach around 3800, while the max I've ever reached on stock GB was around 33-3400.
I do agree with the force closes, though. But, they're pretty rare for me, so I don't mind that much. Also, make sure you backup your phone before you flash. Short of hardware failure, you should always be able to turn back if things go wrong.