based on my experience with with windows mobile in comparison with maemo 4:

first off, pocket ie sucks. period. but if you compare microb web browser from maemo and something like opera mini or skyfire, i'd say skyfire smokes the microb. maemo's webbrowser loads websites prettys low, but it does look like what you see from your computer which is good. downloading the new webkit browser tear still has a lot of problems loading some websites like facebook, but overall faster than microb.

calendar system is practically nonexistant in maemo 4 unless you get something like mcalendar to sync with google calendar, and even that is pretty bad looking. wm outlook calendar is much better.

the email client for maemo4 is much better than outlook mobile. outlook mobile is ugly and buggy whereas the email client (modest? if i remember correctly) looks like a desktop email client and runs much better.

i've never used voip much so i can't help you there