I think they are very handy for IM and SMS, I use them commonly in both. I try not to use them so much in my writing on the forum, or anywhere that I feel I have the time to write in full for that matter. I really do prefer reading peoples posts in full, but if they don't it's no big deal. So whilst I feel abbreviations can be a pain in the ass, they are also extremely handy and ideal for certain situations.