First of all, this is a wide question, as it might depend on your preferences. But in my experience 2010 hangs a lot more tha 2007, sometimes when I open a menu it just hangs (the windows freezes) and after a few seconds it continues working, and that didn't happened with 2007 in the same PC.
OK, I'll add my two cents I use Office 2007 at work but I use Office 2010 at home and out of the two I definitely find 2010 more easy to use & on the eye as well, both versions are essentially the same really. Some of the things I like in 2010 is it now runs the Ribbon UI across all applications where as 2007 didn't. Speed wise, 2007 starts a fraction of a second faster than 2010 on my computer, suffice to say not really noticeable. As already said it's definitely down to personal preference in the end.
2010 has more logical layout for normal user, things are where you would most likely expect them Live paste preview, font preview etc are really nice. 2007 just can not compete sebus