Forgive me for the obvious answer, but I'm going to say it anyways:
It depends.
It's much like asking if third or first or second person is a good idea. It really depends on the work and what you are trying to achieve. Epistolary novels used to be all the rage, and a fair few classics have been written in the style (Persian Letters, Frankenstein, Dracula, Screwtape Letters). Perhaps a bit more information on your dilemma?
ankhirke is definitely right. Everything depends on how well you can pull it off. If it's written well and tells a prevokative story. If it's not written well and if you just can't pull it off then I wouldn't try it.
So its up to you, do you think you'll be able to write it well. If you do, then go for it. If not, wait, practice, and then write.
Never forget who you are, for surely the world will not. Make it your strength. Then it can never be your weakness. Armor yourself in it, and it will never be used to hurt you.
Like the others have said, it depends on what effect you want. If you're just doing it because you think it's cool, your focus is probably off the mark. But if you're doing it to achieve a certain effect, like Stephen Chbosky did in The Perks of Being a Wallflower, then you're more likely to pull it off well. Like with all writing, it has to mean something. If it's meaningless, you're wasting everybody's time.
Apologies for being a little crass. ^^ Best of luck.
Teague
x
"2-4-6-8! I like to delegate!" -Meshugenah "Teague: Stomping on your dreams since 1992." -Sachiko "So I'm looking at FLT and am reminded of a sandwich." -Jabber
I have read quite a few epistolary novels in which the writing came across as somewhat immature, but they were oriented more toward junior high/high school readers. However, one of my favorite books, The Screwtape Letters, is written in this format. I suggest that the character who is writing the letters/diary entries be very thoughtful and in-depth. I am not a good letter/diary writer because I tend to end up either getting preachy or describing things like you would in an ordinary novel.
Alternately, you could always write a diary entry, and then have a "flashback" where you describe what happened in third person. I used to read a series of books that did this, and it seemed to add more depth and interest to the novel.
I definitely think varying the format of novels is great if not done excessively (of course, if the entire novel is letters etc then that's okay). The problem is that real letters often deviate majorly from things that are important to the story, and to get that sense of realistic writing you need to be able to have these deviations without making the reader bored.
Another caveat I have (which applies to modern day settings): some authors make teenagers have way too many discussions on IM. This is annoying to me because, a) teenagers I know in real life don't go on IM all that much and; b) it is still a novelty to many, and often discussions revolve around the emoticons/chat program itself, rather than anything else. Even when it does change, it's most likely a continuation of a real-life conversation, and the IMs wouldn't make sense without having heard/read the other conversation. Of course this will vary between schools and groups and even individual teenagers.
Second IM/modern technology-related caveat: people don't use abbrevations as much in real life as in novels. The only ones I've really seen are lol, cos/coz (because) and, um, that's all. Abbreviations are usually more about saving space than saving time, so are most often used in texts to keep the cost of the text down to single (longer than 160 characters and it costs twice as much). Because there is no cost associated with IM, it is likely to have less abbreviations than a text message.
"You are altogether a human being, Jane? You are certain of that?" "I conscientiously believe so, Mr Rochester." ~ Jane Eyre