The following was my long comment to Droid vs. iPhone 3GS: An Update article on Technologizer site.
Interesting! I never had Verizon Droid, but I used my beloved ADP1 (HTC Dream, G1) + Cyanogen mod on AT&T EDGE for one and half year and finally switched to Nexus One on AT&T 3G.
Yes, ADP1 was slow and clearly lacked memory and CPU power, but I still loved it and it became better and better with new apps and Cyanogen updates. It was necessary to kill some apps periodically and to reboot phone periodically, when it became to freeze or to behave irrationally.
Since I bought Nexus One I never had a real need in killing apps. It almost never becomes slow or strange due to too many apps running. In rare occasions, I prefer to reboot the phone, as Steven suggested above. Overall, everything is really stable and solid. Because of that, I didn't want to experiment with installing Cyanogen for Nexus One so far.
Yes, there are few quirks in this phone too (show me the electronic device which doesn't have bugs, please.) For example, after taking Nexus One out of a dock it sometimes starts to respond strange. But turning screen off with a top button and then back on and unlocking the screen fixes it immediately. So, it's quite minor bug, which hopefully they fix soon.
Otherwise - rock solid. I never had iPhone, so I cannot compare, but to me, Android's interface of Nexus One is absolutely sleek and intuitive.