I've been experimenting with the lectionary since Christ the King Sunday, albeit with modifications. We have two formal Scripture readings at my church, not four, so I can't use all of the passages, nor do I think that it is a good idea to do so.
The first advantage apparent to me is that I don't have to think too much about what to preach on. I have a list of passages and choose from them for the sermon. I don't, however, always stick to the lectionary's restrictions. For last Sunday, my lay speaker read John 1:1-17, which I also wove into my sermon on Matthew 3:1-12. I used the lectionary Pslam 72 for our responsive reading, but it really didn't fit with my sermon.
I have been to Lutheran and Episcopalian services where the sermon was based on all four readings, and saw an immediate disadvantage: in a sermon thus constructed, the preacher must interpret the passages in reflection of each other, thereby skewing the exegesis.
I like the idea of my preaching being ordered in a thoughtful, not erratic manner. And I also like the idea of keeping close to the liturgical calendar. But I dislike the lectionary's omission of large sections of Scripture. For example, one of my better sermons is from Habakkuk 3, which is not covered during any part of the lectionary cycle.
I would be interested to see a lectionary that encompassed the entire Bible, even if it took a decade to complete a cycle.