You two will be mentally in tune. You'll wonder to yourself -- maybe even aloud -- how this person can know so much about you. They'll be highly observant of you from the start, able to tune right in to your subtle cues and to draw an accurate picture of your individual nature from comparatively little information.
This is a wonderful gift for you to enjoy, but it shouldn't be taken for granted. You may come to expect your friend to understand you so well that you won't think it's important to verbalize your thoughts, needs and feelings anymore. In general, though, there's not much of a downside to this kind of communicative bond. You'll feel heard and understood, which will translate naturally into support and acceptance. And if you make a bit of extra effort to put things into words when misunderstandings do crop up, you'll be able to sail through the rough spots and get back on even ground again.