Apps can run in the background so you can log your cycle whilst emailing / texting etc.
You can do this on iPhone too.
I just went back to an iPhone after about 18 months with a Galaxy S, i’m happy to be away from android, things that sucked (won’t be the case with all android handsets i’m sure but enough to put me off another one) were terrible GPS performance which was a known issue with galaxy S. Slow apps, not written as well as equivalent iPhone apps. When switching to wifi at home, internet would often be really slow, slower than 3G. Lack of timely updates and likelihood of future updates. I’m sure there are more.
Things I miss, pull down for switching wifi on off. I liked the calendar widget but the new pull down in iOS5 has upcoming entries, it would be nice to be able to see them on the home screen though.
edit: lastly size, the galaxy S was about the right size, the top android handsets are just too big now and I really like the smaller size of the iphone.