i have been self taught and am at a reasonably stable level but i do not consider myself having mastered the instrument as there is alot to learn. i still believe in getting a teacher for help
it is indeed possible to master the guitar being partially or totally self taught. the only thing, however, is that when you teach yourself to play guitar, bear in mind that no one is guiding and following up with you on learning the instrument. and you may not notice it, but bad habits in guitar playing will eventually start to form if you aren't aware of what you are doing.
BUT if you are aware of what you are teaching yourself and applying the correct techniques and theory to your playing, you can then consider yourself having mastered the instrument.
so remember; practice hard, and practice SMART.
cheers =)