I want to be a linguist focusing on conserving languages. Should I do it?

What my betters have said, with both the pros and cons from Don Grushkin’s answer.

Be aware of the following constraints:

• Don’t get too caught up in what language you work on. A friend of mine came to Australia to write a grammar of an Aboriginal language, any Aboriginal language. There’s 20 healthy languages left, and they’ve been recorded—it’d be detail work now. He ended up going to Papua New Guinea. You go where your prof sends you. Which is why the wise thing to do is to chose your prof before your prof chooses you.
• Don’t go in thinking you’re a saviour. Language communities particularly in places like Canada and Australia are very attuned to that attitude. You’re a partner to them, and the community owns the language, not you. You will need to be humble, and you will need to run lots past them. There will be post-colonial resentment to face. And your name may not be up in lights in the grammar and dictionary as much as you’d like. This happened to another contemporary of mine (who has since abandoned linguistics); I know I have too much of an ego to deal with it.
• Someone in a paper I read sneered about linguists ending up doing feelgood welfare work for language communities (see many of the other answers here). If you’re working on language revival, be aware that your aspiring speakers of the language are not professional linguists, and you will have to dumb things down.
• You know how the CIA did not have enough human intel in Afganistan before 9/11, because “noone wants to sign up for a lifestyle of dysentery”, as they said at the time? Well, same with fieldwork. There is a reason that most of what we know about Papua New Guinea languages comes from missionaries. For that matter, there’s a reason why Vanuatu, which has an even denser concentration of languages, is not well-documented linguistically: the government has banned the missionaries. 
• The language communities you work with may be an infighting, petty, small-minded bunch of loons. You may get attitude like, “If you speak to that side of the village for data, noone from this side of the village will ever speak to you.”
• The linguist colleagues will definitely be an infighting, petty, small-minded bunch of loons. I wrote a jeremiad about my personal experience with obtaining a linguistics degree; including having to “step on corpses” to get an academic job. It will be worse in anthropological/fieldwork linguistics. Moreover, there’ll be less places you can get a job. I am from Australia, which is fieldwork country. The only countries our linguists are on speaking terms are West Coast US, Germany and Netherlands (thanks to the Max Planck Institute), and a couple of guys in Japan.

Don’t let this talk you out of it. At all. Just be aware—even more so than anyone considering a career in linguistics in general. Be prepared for some disillusionment.

And from what I gather from those I know that have stuck with it: be prepared for some life-changing, life-long friendships too.