For all of my life pretty much everyone has known me as Kareem. I've had a few nicknames, but nothing that really stuck.
And then I went to Brazil.
In Portuguese, the letter "M" is a little different. At the end of a word, "em" is pronounced more like "ein" in English. For example. "também," the Portuguese word for also or tambien in Spanish, is pronounced more like "tambein". That turns Kareem into "Kareein". If that seems really weird to you, it should. Between the different "M" sound and double e's (which never occur in Portuguese), this is about the equivalent to Przemysław. Most figure it's a misspelling of Karen.
It became apparent that I needed a nickname. In Egypt, Kimo is a common nickname for Kareem. My cousins had once tried to call me Kimo, but I didn't like it. Apparently I don't take to nicknames all that well. While not exactly a common name in Brazil, it's at least pronounceable and relatively easy to guess the spelling in Portuguese. At Google, the login name "kimo" wasn't available, but "kimos" (Kimo + S from Shehata) was. Getting a good login name at Google now is very hard, because they never recycle them. It turns out that "kshehata" was very difficult for almost everyone to spell, and you use your login name everywhere. So the solution was obvious: use Kimos as both my login name as well as my nickname at Google and for anyone who speaks Portuguese.
So what name do I go by? Both. Most of my friends in Canada or that speak English will call me Kareem because it's what I grew up with and it's more common there. At Google or in Portuguese I'm Kimos. I already have emails addresses as Kimos, and I'll respond to both. Go with whatever is easiest for you.