My friend and I are learning Japanese. For a time, all we knew was 私, but after we learned more words, she started to use あたし when referring to herself, and I began using 僕. What I'm wondering is, how commonly are these used?
あたし is quite common for females, but 僕 is not that much. In my feeling, 僕 has some romantic sense, so using 僕 when you talk to girls should be no problem at all. (Note that I use romatic sense here is for non-family members, and non-closed friends)
Japanese use 俺 a lot recently, and here is a report about usage about 僕 and 俺
第１期 - 1895~1935, 第２期 - 1966-1991, 第３期 - 1991~1998
And this is the another report about 僕 vs 私 at business/work taken at 2011, results in around 20%~20% people using 僕.
This is a very difficult question to conclusively answer. "Common" depends on where you are, who you're with, and what the context is. The word you choose to refer to yourself often changes depending on these factors. If "common" means, "Does a significant percentage of Japanese use these words regularly?" then yes, everyone will tell you they are common. But does that mean you can use them all the time? Of course not. You have to adjust to whatever situation and context you're in.
It depends on who uses it. あたし is very common with women. Gay men also use it (sometimes for a joke, like Tanoshingo). If you watch Japanese TV, you'll hear it everyday!
僕 on the other hand isn't that popular within young guys. They tend to use 俺 in informal speech. However, instead of using the too formal 私, they might use 僕.
(Young) people (or kids) using 僕 might also be called 僕ちゃん, because it has some kind of a cute meaning.
僕 tends to have a more polite feeling, a more "educated" feeling.
In Tokyo, among the people I speak with, this is mostly a matter of age.
あたし is simple. In Tokyo, younger (er) women, say 30 and below, use this all the time in casual situations.
僕 is commonly used by my friends in their 30s, whereas friends in their 20s tend to use 俺.
I use both depending on which group I am speaking to.
Also very common is to refer to yourself using your own name. This feels weird for English speakers at first but is normal in Japanese.